some things to keep in mind
-different manufacturers put the mast tracks in different positions both by brand and by model and size. example for freestyle boards some years ago, RRD had their mast tracks several inches further forward than JP & Fanatic, so if you switched back and forth from say a Fanatic Skate to an RRD Twin Tip with both boards set mid track, your stance/sail geometry would be totally different. Also, depending on the brand, the mast track on a new school wave board is likely to be a little further back than on a freewave board or more traditional wave board. also, sometimes the brand puts the mast track in the wrong place. for example the 2012 Fanatic Skate 99 TE... I had to run the mast track all the way back to get the board trimmed correctly. For 2013, they kept the shape the same but moved the mast track back about 3 inches, so now I was running almost dead center. This was a really long way of saying, don't just stick the uni in the center and trust that it's perfect! Lots of personal preferences for mast track location and trim can vary with sail size, sail type, stance, etc...
-mast track adjustment range: the Chinook 2-bolt plate has a very wide bolt spacing that limits how much adjustment you can make. Streamlined makes a 2-bolt system with the screws much closer that gives you about 50% more adjustment range (although streamlined is harder to get in the USA these days). Duotone makes a single-bolt with detachable plate system that gives you the full adjustment range of your mast track. I've been using this for two years now, and really like it. My foil board has the uni slammed all the way to the back of the track, a position not possible with 2-bolt baseplates.
-mast track measurements marked on the board are to the end of the tail: i prefer to measure to the midpoint of the fin or so. For simplicity's sake, I measure to the power box screw hole. I do this because boards have big variance in fin position relative to the end of the tail. STBs the fins are quite close. 2007-2009 vintage Fanatic Freewaves had a lot of tail sticking out behind the fin. Modern wave boards have this distance shortened quite a bit over the last 5+ years. Another way to do this is to measure to the back strap (useful with some multi fin boards like quads where your back strap is standing directly over the two rear fins.). with power box boards, the rear strap usually screws in right near the leading edge of the fin.
-with many power box fin-equipped boards with inboard straps, I like to set the back strap all the way back, set my mast base relative to the back strap/fin hole, and then set the front straps to get my preferred stance. The fin is the one thing that doesn't move, so I basically set everything relative to that and make adjustments as needed. Some shapers and fin designers have suggested this method also.
-my STBs and Skates, just going by the factory numbers to the tail, I set in the 120-121cm range, although these boards don't have a ton of length behind the fin. also, I am admittedly a track back kind of guy and sail smaller sails with high and forward draft.
Hope I didn't bore you guys to death! YMMV!!
