Quick Force Point Question

Can I spend a Force Point after my opponent has already resolved his Block/Deflect roll?

I ask because last night I rolled a crit but the total roll didn't beat the Sith's Block roll. I said I wanted to spend a Force Point but my GM said I couldn't otherwise defenses would be meaningless. If I wanted to spend a Force Point I should have rolled it along with my initial attack.

This doesn't seem right to me. I thought you could spend a Force Point whenever you wanted.
The rules on when you get to roll Force Points are a little fuzzy, but I imagine most groups use a variant of the old RCR method, which required you to roll for your Force Point bonus before knowing the result of your roll or check. In your GM's shoes, I would not have allowed you to roll after knowing the Sith successfully Blocked, and even though I would let you roll after rolling your own d20 (but before knowing the result), you likely wouldn't have, having rolled a 20.
As I understand it, you should have been able to spend the Force Point. However, the Sith would also have been able to spend a Force Ponit after seeing your booster roll.

Given the lack of specifics in the rules, this has lead to a lot of House Rules.

In the end, I guess it's always going to be the GM's call.
Actually the rules say that you can use a Force Point on your turn except in 2 circumstances. 1) To keep from dying. 2) If you have the Force Readiness feat. So the Sith could not use a force point to help with his block/deflect.
You see your roll (a crit- yey!) and it is then when you have to decide whether or not to spend a Force Point to increase the roll. You can NOT wait until after learning that little old Mr. Sith just rolled well enough to block your hit because then you'd be adding the die after the ability has been resolved.

Actually, I may WOULD let you add a FP to your roll after the Block is resolved. Of course at that point your attack has already been blocked so I'm basically just letting you spend a FP for no effect.
Im my group: NO.
Normally, it is easy to decide whether or not to use a FP. For example, everyone knows that the DC to lift a large object with move object is 20. If your roll + modifier yeilds a 19, you know you need to spend a FP or fail to lift the large object.

From a roleplaying standpoint, this works because a Jedi should be able to sense whether or not he has summoned enough force energy to lift the object and if he feels he has not, he can draw more deeply on the force to compensate (by spending a FP).

In this case, however, your character doesn't know exactly how much effort it will take to overcome the opponent's defenses because he cannot know what the result of the block roll will be. In this case, the character must decide how much effort to devote to the attack as he attacks. He cannot wait to see if his attack is blocked and then decide to add additional effort to his swing.