Instead of measuring it exactly, why not just take a piece of wood of some length and use it as a spacer. Place a stake in every spot, then stand back to see if everything looks fine. YOu can adjust to smaller in between spaces in the inner curve and just a hint wider on the outer curve.

One on each side of the path? Take two cardboard boxes and place them until they look right from different angles.

tj

I poke sticks in the ground then stand back, circle the area a zillion times, move the sticks around, repeat. Only takes about 2-3 years to get it just right.
:) Laura

The important part of these suggestions is "stand back and look from different angles (a zillion times)"
Don't line them up exactly opposite each other. That is static and the eye will jump from plant to plant (boring) The aim is "flow" just like the path. Really you're dealing with three flowing lines or spaces: each side of the path and the path itself. You will see that if the plants are exactly opposite one another the eye will read the spaces inbetween as "holes" but if staggered somehow the eye perceives a "whole" Hard to explain but when you see it you'll know it.
Stand in every spot you will view the path from: from the public sidewalk, the door, the driveway, where the path begins and ends. Funny how plants can disappear behind one another from different angles.
. When I was laying a flagstone path the best vantage point for planning was an upstairs window. Again, seeing the "whole" was very helpful.

Are you using only lavender? How is the interest in the path maintained when the lavender is just beginning to grow or not in its prime bloom? In traditional gardens of the affluent English ye olde familie gardener would change out plants by the season so something was always in bloom or show.
great question. I've done it but never tried to articulate why one way works and another doesn't.
idabean

Thanks for all your imput. I won't beable to get the lavender or whatever I decide to do until spring. . . The path runs the length of about 72'. I have been planting the beds with conifers and cottage plants such as hydrangeas, roses, and various perrenials. Any ideas on maybe a three season plant that could be used to add some structure to the beds. Your points are well taken, thank you.

"Any ideas on maybe a three season plant that could be used to add some structure to the beds"

Pat_tea, I don't know what zone you are in but it's likely warmer than my zone 3. I have a curved path through a perennial bed and have used Dusty Miller with Munstead Lavender behind and between on one side of the path and the Dusty Miller with perennial flax on the other side. I'd have used all lavender but the flax is hardier and I may have to replace all the lavender with flax later if some don't survive the winter. I've been quite pleased with this and would post pics except I've been too busy gardening this year to deal with the pics I've taken. The Dusty Miller is not successfully perennial here but certainly looks very good for 3 seasons. Might be perennial in your zone. Perennial flax is really only 2 seasons as needs deadheading in August otherwise one has a million of them. Lavender is still looking reasonably good here and I've been lucky with it as it's not reliably hardy in this zone.

Another plant I like along pathways is Walkers Low Catmint. I first saw this on Freda's blog and loved it. See the link below and other catmint links there. I've propagated my WL but still don't have enough plants to edge the path. Will take more slips in the spring to make more plants and eventually may do the same as Freda.

OK, back to the original question as to how to place plants evenly on both sides. What I did was to measure to find the center of each side of the path. My path curves so these are not equal measurements. Then I found the middle point of each side and marked it. Then I took a stake which was approximately the distance I wanted the plants spaced which was *about* 18" and marked out from the center. I then fiddled with it until it worked out fairly well at each end. I'm not a perfectionist in my garden so close enough WAS close enough. This particular perennial bed is somewhat raised from the lawn so I couldn't go too close to the end of the path but IMO it looks good.

Hope that helps. :)

Here is a link that might be useful: Have I Told You Lately That I Love Catmint?

Thank you Lukygal. I was thinking about walkers low and in fact had settled on it until just recently when I saw it in a friends garden. It was yellow in the center and not looking good so I had ????

I can't wait to see pictures of your garden. I know what you mean about being too busy to take pictures. I have been trying to document this overwhelming project but I have been so darn warn out I haven't taken any for a few days. Please post pictures when you can.

Pat-tea, oh please try to post pics. I would love to see the conifers, hydrangeas, etc. that you have so far. I am always looking for inspiration!