So... how much is "enough" XP? ???
It's on the company spread sheet. But here's how I've broken it down:
A mechwarrior with skills of 4/5, it takes 5 XP to improve your skills.
Once you've improved to a 3/4, it takes 10 XP.
Upon reaching 2/3, it takes 15, a 1/2 takes 20 and so on.
I consider any SPA worth 2 or more points as if you've reached the next tier of XP requirements for skill improvements. If you all want to use the alternate XP option, then a pilot is ineligible to purchase an SPA until they've improved their skills to a 3/4, but if you want to do the roll of an 11-12, then any pilot (regardless of skill level) can get a SPA.