Yes i have a few seats. I have a Standard tillett in ML size, a Soft tillett in ML size and a Standard Tillett in MS.
What chassis, approximate height etc? The thing i've found with seating pos. is not to overcomplicate it. Firstly finding the right seat for you is essential, ie. for me the tillett T12 was the best because im a tall driver. Now when your fitting it, you want it probably just underneath the chassis rail (5mm). And you generally want it quite leaned back (especially if quite tall). Now all there is left is front to rear, now basically my best advice would be to put it in position. Sit in it, and see how it feels, if it feels nice and comfortable, your off and running. Note, that if a specific issue such as no rear grip arises, you may be able to move the seat back 10mm.