Under the normal rules, a sorcerer just has to keep one of his spell slots of the minimum level in reserve to continue to benefit from the feat.
But with spell points, you don't have spell levels to keep in reserve: As long as you still have your last few spell points, you're capable of casting that spell. I don't think increasing the number of spell points you need to have available will provide much of a fix: either way, you keep the ability until you're very close to the bottom of the barrel.
What I would suggest is this: You have to commit your spell points to casting the spell in question the first time you use your reserve ability, and the spellpoints can be used for no other purpose.
So, let's say you're a 10th level wizard with the fireball-based reserve feat and the polymorph-based reserve feat. with a 20 int, you've got 98 spell points. In the first combat, you decide to use your mini-fireball. You want it to be powerful, so you put 9 spellpoints into it, giving you the ability to deal 5d6 damage every time you use the attack. Those 9 points are spoken for, and you can't use them to cast anything but fireball. The next round, you decide you want to use the reserve version of polymorph, so you put aside 7 more spell points. That leaves you with 82 points you can use to cast anything, and 16 that are reserved for specific spells.
The next combat is a bit tougher. You've spent the first three rounds already casting your best spells, so now you only have 52 general points left. on the fourth round, though, you decide you need to cast polymorph. So you spend the 7 points you had put aside previously, and can no longer access the reserve version of polymorph. Now you have 52 general points, and 9 points reserved for a fireball spell.
The start of the next combat, you decide you want to use the reserve polymorph effect, so you reserve 7 more points. That's now at 45 general and back to 16 reserved. Then you spend the next several rounds casting spells, and you wind up with only 4 general points left (plus your 16 in reserve).
Before you can rest, there's yet another combat. It's a big one, and all you have left is four points. You decide you want to cast a fireball. You could spend the 9 points you have in reserve, but then you'd be out your reserve ability, and you doubt that one fireball's going to be enough. So instead, you spend 4 points from your fireball reserve (leaving 5, the minimum to cast fireball) plus the 4 points from your general pool to cast a 9d6 fireball (the reserve feat gives you an extra die). Now, you can continue to use your reserve fireball, but it will only do 3d6 points of damage.
