In my long RM career, I have been all over this with a variety of rules.
Guidelines: I prefer to use first aid, second aid and surgery. First aid covers stopping bleeding up to five hits per round and applying herbs. Second aid involves all setting bones, treating 3rd degree burns and stitching shut wounds that bleed 6-9 hits a round. Surgery is used to remove objects (arrows, shrapenal, etc), stop bleeding over 10 hits per round (essentially clamping off/repairing severed arteries/veins) and repairing shattered bones.
when a lay healer must perfom a surgery to complete a spell, you can make a roll with the appropriate skill. SM has a very nice table that can be used for "repairs" that tells how long the mnv will take and cost in parts. This info is easily extrapolated into cost for the lay healer. All lay healers should have a surgeons kit. Base cost is 17sp in my game. The lay healers spells should allow the use of anything handy to help perform the surgical task.
If the roll "fails", this typically recovery is longer with a larger penalty for the wound equal to how much the mnv was failed by. So a mace shatters a hip and with gear on hand the GM determines the surgery will be hard. The roll results in near success, short of failure by 7. The wounds penalty is increased by 7 and recovery time is incresed by x2. Partial success would be x3. Failure x5.
For spells that result in instant rcovery, mnv failure indicates there is now recovery time. Minutes for light wounds, hours for medium wounds and days for severe wounds.
A little bit of work fleshing out the healing rules will result in many good role playing returns. Players get to be "wounded" and all sorts of new NPC's and adventure material will emerge. I cant see what i am tyyping so I close with an apology for my errors and not posting more.