John, I've never heard of that rule. In fact, I'd be worried if all I had was 2 feet longer. Think about it. The rollers end at the transom, so even allowing a foot overhand for the roller support that makes your trailer is only 23' at it's longest, which, for a V20 means your trailer only has 3 feet of usable tongue length max. On to that you have to mount a trailer jack, maybe a spare tire, a brake actuator, winch post and still allow for side to side trailer movement when making a turn either going forward or backing up. So, IMHO 2' just isn't going to make it. I'd be much happier with 4' minimum. (Oh, and don't forget you have to allow for the possibility of shifting your winch post as well as your trailer axle(s) to arrive at the proper tongue weight for towing).
I guess what I'm saying is that IMHO it's far better for the trailer to be a little longer than shorter.
As for the bow pulpit question, I would think you measure from a plumb line dropped down from the furthest most point on the boat, which means yes, you count the pulpit. (Don't forget, that pulpit is going to hit the *** end of your car if the trailer is too short).