A real tru tile setter can float there tile FLAT. We used to do this with wet setting on bigger tiles, Now we have medium bed thinset mortars that can be used up to 3/4" of depth under the tile so we can float to some extent with that. Floors are not always level and thats OK we are more looking for FLAT and in plane. If you start with a FLAT substrate that meets tolerances for TCNA/ANSI I dont always use lippage tuning. It comes down to a job by job basis, The quality of the tile, and many times the material budget. The one and only excuse for lippage is the tile material itself. If its warped / bowed etc. If the substrate is not in tolerance before I tile I know I have to float. If im floating I make sure its within 3/4" depth for my thinsets or I will sometimes wet set in which case I will level as I go.. Usually I fix the substrate first or skip on the job if they dont want to do that $$. But you dont get a floor with a bunch of lippage and I dont get a bad rep. Actual well installed marble floors can be polished and some lippage can be mandated. Its not cheap. We had a recent post here by one of our skilled veterans in it he speaks to bending tile with lippage tuning to force stagger patterns..... I have on occasion taken jobs with bowed tile and forced them somewhat into submission with Lippage tuning like that and it can really help (but thats not a spec and adviseable to try for many) lippage tuning doesnt LEVEL a floor it helps keep the tile flat and in plane. Leveling happens underneath and before tiling or via wet setting/floating. I have set 24x24 tiles with no tuning system just fine so you dont neccesarily NEED it always. Off topic: Its a great post you have here, we are in a market these days that tells people do it yourself, watch youtube and HGTV ....... as such weve got a bunch of novice diy tilers turned craigslist PRO that cant get the job done right. Usually half the price of A pros price and Homeonwers are getting the brunt of it while the entire tile industry remains largely unregulated on local levels and gets a bad rap daily here on Houzz... "Help Total tile debacle" " Why does my shower floor look wet " etc etc. But we dont get the full story, People claim to hire tile pros when in fact they show us no proof or paperwork, no company names, where they sourced the install etc. and I think a lot of what we see here on Houzz is folks hiring cheap labor off craigslist and trying to manage their own renos' getting in way past their depth and then coming on houzz complaining and not sharing all the facts thus making the entire PROFESSIONAL tile industry seem shady.. Not all of em but im sure its a serious number and I see it here in my own backyard all the time. Is that how you ended up here asking this :):)???

So is there a formula? Two thirds the width?

Definitely personal preference - I personally like the words. When you take it to be framed, they can show you both ways if that helps you decide.

I tried to post and found all my air-dropped files were in a format that would not post. Will have to find time to redo them. But my art is so different -- some "fine" and a lot of contemporary and folk art. This was the only one that worked and I took it down towait, but in case I don't make it back (I am behind on an 8 week course and trying to get caught up) -- this will be my one contribution. This is titled Angels from Above and was painted in response to the attack on 9/11. It hangs in my LR where I have an Americana folk piece, a new still life that may not have found its permanent home and some other contemporary art. It's an eclectic grouping with the link being rich saturated colors. The sofa is midnight velvet and the chairs are natural linen, both treated as neutrals. The background is an inset above the mantle that was painted for another painting when this was in DR. It isn't terrible with the painting, but were painted the walls and now I want to pick a new color for the inset and different painting.