Playing Mutant Future, pulling rules out of my hat

Mutant Future is super simple. The rules are basically:

Roll to hit.
Roll damage.
Roll to save.
(Optional rule: Roll below ability if you try stuff).
And also technological rules.

But what if something happens not covered in rules? Generally that's where roll ability steps in. But here's some rules I used in this game session, I pulled out of my hat during the gameplay.

Sneaking And The Guard
Character tries to sneak and avoid guard's vision.
Player rolls 1D6 + Dexterity modifier
Referee rolls 1D6 + guard's Intelligence modifier

Higher roll wins (stay hidden versus spot). Draw? Didn't happen (character won so guard didn't spot him) but maybe roll again or something.

Sneak Attack From The Behind To Slice Throat
AC 9 (unarmored), roll to hit but D20 x2. If the result is high enough (double what needed) the opponent is dead. Bloody but silent. If hit 2x damage. If a miss 1x damage. If you roll natural 1 sucks! Guard notices you!

Great Quality Sniper Rifle
If you can aim and your target is not aware of you, you either:
Get +4 to d20 roll or x2 damage (sniper rifle damage is d12 so roll 2d12 if you choose damage bonus).