I have a Heavy Line Rate that is roughly 20% greater than the standard rate. I allow 2 days extra for an engine job, less for a trans. That allows time for other work that comes in. Then on an engine job it gets new hoses, belts, filters, plugs, etc. Transmission I try to sell new axles. In all if I schecdule right I can get those done and not run off my good regular customers.
I dont do a lot of engine work...couple a year. So many "shed shops" around here that will do it for half of what I quote. Mostly my regulars that have an older car and dont want car payments. Last one was a 2010 Sport Trac with a 4 cylinder. Was at 200k and burned a valve. Client opted for a 40k used engine over head reapir. Took 4 or 5 days and never missed a regular customer. Cant say I enjoy doing them but it is revenue and good if it is worked right. Just my 2 cents.