The cultivar, the length of the interstem, and the local climate are going to make enough of a distance to screw things up. If you're planting 1,000 trees you should find someone locally who has planted the same rootstock, interstem length, and cultivar combination you have in mind.

I would suggest that you look to G.11 or Bud.9 (as a rootstock, not an interstem) unless you have really bad soil. They are cheaper and more readily available than interstem trees.

You might also consider M.111 or seedling at a wider spacing if you're willing to tolerate a larger tree.

I'm grafting the interstems myself to save money. I only plan on planting around 100-125 trees. I know the spacing for plain Bud 9 but I want to use the interstems to avoid trellises or staking the trees and to keep from having to irrigate the trees after their first year except during a drought. I plan on using a 10 to 12 inch piece of bud 9 interstem on the Bud 118 rootstock. I like free standing trees but a lot of the smaller semi dwarf and larger dwarf rootstocks have problems in my soil, they're not free standing or they get burrknots. Plus the Budagovsky rootstocks are not patented so I can propagate them freely.

See article below which confirms what I have heard - the interstem completely dominates. There are some issues about how long the interstem is, I would make it plenty long unless you find information indicating it is OK to make it short. Anyway based on these data I would use Bud9 spacing for your interstems.

Scott

Here is a link that might be useful: interstem info

Yup, they'll be small trees. I would still expect their ultimate size to depend somewhat on cultivar. With Honeycrisp you could sure plant them 6 feet apart N-S. With a more vigorous cultivar, I would think you would want somewhat more space than that. You would want more space E-W than N-S to allow sunlight penetration.

Depends on your goals, too. If you have room you can always plant them wider apart and leave some room between them.

I would say 10'. Seems to me that trees are more likely to grow bigger than their intended size unless you plan on keeping after the pruning.
Even if you plan on keeping after their pruning plans can often get put on hold, but growing seasons don't.

Here is a link that might be useful: Iowa State interstem study PDF

Scottfsmith,
Thanks for the link to that article. It answered my question perfectly. I'm using the Bud 118 rootstock for its resistance to drought and its better root system along with its resistance to apple scab. I'm using Bud 9 for its dwarfing and its fruit quality. I'll bud graft onto Bud 9 rootstocks I have already, then when they're dormant I'll cut off the grafted bud and 12 inches of Bud 9 stem to use as the interstem. I'll keep them in the fridge until I get my Bud 118 rootstocks from Treco this winter. This should save me a year of growing time by budding the interstems now.
I've decided to do a N-S spacing of 8 feet and an E-W spacing of 12 feet. My orchard spot is a gentle South facing slope near to, but not on top of the mountain. It's a strip of land about 75 yards wide N-S and 200 yards long E-W. It's sheltered on the East side by the house and the woods behind the house, and on the North and West by woods and windbreaks. It gets a constant breeze in the summer but stays pretty calm in the winter. Thanks again everybody for all your help,
Paul