Height is pretty obvious, just from the center of the BB to the top of the saddle. Keep the same number with the new saddle, maybe up or down the seatpost.

As for back and forth is hard to tell because who know where the new saddle wants you to seat but pretty much probably the guy will measure their length and then adjust depending were the tip of the saddles are falling in relation with the center of the BB shell.

Probably all of this will makes sense now, right?