I have never had a problem locating the seat because the software I use to design my kayaks (Kayak Foundry), does a very good job of positioning the cockpit and I have a small amount of adjustment available in the seat mounting.

The technique of sitting in the kayak in a pool works for getting close to the right spot, but it often won't be perfect.

The greater the amount of power applied to the paddle, the more the trim generally changes. The position suggested by static tests is fairly close for leisurely paddling, but often a bit more speed is available if you move the seat forward a bit from there. In fact in a K1, which is under full power all the time, the ideal seating position is a long way forward of where you would want it for recreational paddling in a sea kayak.

Of course moving the seat forward in pursuit of a bit more speed usually comes at a cost. Some kayaks become a little less stable as they are trimmed further forward and some weathercock a lot more with that trim.