A variation on the idea of just using the % success/failure rate of your skill level is to first have a few dice rolls that can modify this; I suggested such a system

here. You can speed things up and get roughly comparable results by instead using 3dF, as I noted later in the thread. It typically requires a calculator (or a player with the Lightning Calculator Advantage) to use, but I'm pretty certain most of us carry one of those around at all times anyway (as an app on our phones).

EDIT: Let's go with an injured PC laying in a TL 8 hospital for 30 days. The PC has HT 10, and the hospital staff is working with an effective Physician 12. Normally, this would require 30 rolls against 11 (HT 10, +1 for Physician 12+) for natural recovery

*and* 30 rolls against 12 for assisted recovery. Under my suggestion, we can either roll these separately, for 3 rolls each (total 6 instead of 60), or even just apply the same adjustment to each (for 3 rolls instead of 60 - or 30 if above we simply made a single roll and checked it against both HT and Physician).

For this demonstration, I'll do both. For the natural recovery rolls, I'm getting 13, 12, and 16 (bad luck on those rolls), for a total of -9; that puts the character at an effective value of 2, so he fails to recover on his own at all (fortunately, Critical Failure doesn't do anything on natural recover rolls, so we don't need to look at that). For the assisted recovery, I'm getting 8, 8, and 11, for a total of +3; that puts things at 15, or 95.37% success rate - that's 28.61 successes. With our initial skill 12, MoF 5+ (a roll of 17 or 18) is needed for a Critical Failure, but we hit a 0% failure rate at just MoS 3, so we're safe. A Critical Success, meanwhile, requires MoS 8+ (roll of 4 or lower); that corresponds to 7 on our table, so 16.20% of our rolls - 4.86 - are Critical Successes, each of which restores 1 additional HP. So, our injured character recovers a total of 0 + 28.61 + 4.86 = 33.47 HP, which rounds down to 33 HP.

Note using 3dF (which if you lack dF - like me - you can make do with setting 1,2 as +, 3,4 as 0, and 5,6 as -) makes things markedly less swingy. Even the worst roll above, 16 (which was 5-5-6 IIRC), would have only been a -3, letting the character heal for 7.779 additional HP, while the best, 8 (which was 2-2-4) would have been good for a +2. Rerolling with essentially 3dF, I'm getting +0 (2-4-5) and +2 (1-2-3). That puts the natural healing at 11, for 62.50% success - 18.75 (natural healing doesn't care about criticals in either direction, so no need to check for that). Our assisted healing is at 14, for 90.74% success - 27.22. For skill 12, you need MoF 5 for Critical Failure (which isn't an issue here, as we hit 0% failure at MoF 4), and MoS 8 for Critical Success; the latter corresponds to 6, so 9.26% - 2.79 - heal an additional 1 HP. Total healing is therefore 18.75 + 27.22 + 2.79 = 48.76, which rounds up to 49 HP.