RoCo 3 has the Superfast Hits Multiplier Chart. You could add the number of Ambush ranks (or maybe 2x or 3x Ambush ranks?) to the number you look up on that chart. That increases the chance of doing double, triple, etc damage.
You could also, say, divide the number of Ambush ranks by 5 and inflict that many extra d10's of damage, plus the remainder. E.g. if you have 22 ranks, add 2d10+2 damage. This means it always helps but takes away some excitement.
You could roll an Ambush maneuver and on success give x2 damage, absolute success x3 or x4.
You could forget the damage and focus on the chance of instant death. Roll an Ambush maneuver, on success the target makes an RR or dies. Not my preference but just tossing some ideas out there.
Also, I'll just note that personally I use software to track combat. Keeps the detail, speeds things up probably more than switching to a system like this. If that's your primary concern it might be worth looking at one of those options as well.