Regarding the 70%: If you were selling hard goods, you would consider the cost of the goods plus storage and other overhead, then add the profit margin that you require. The same should be true of a service business. If you can meet your margin after figuring in the cost of the job ( overhead including your equipment cost/depreciation/storage, transportation, wear and tear, etc. PLUS your time) than take it. If you can't meet your margin- don't.
The subjective part is your time. All else can pretty much be figured out. The question is: what cost do you set for your time alone? THAT'S the place where some get fuzzy. ( "Well, it's better than sitting home watching TV"). Bad news folks- We have a finite amount of time in this biz, and for life in general. Example: Say you know you're going to have 20 years in this business. Now pretend you're going to have two 4 hour gigs per week with 2 weeks off for vacation. This means that you will have 8,000 hours of chargeable time ( 400 / year ) to make your living AND build a retirement fund. What's your yearly income goal? ( Your living expenses plus money for retirement?) What's your retirement goal? If you have exactly 8000 hours to make those goals, what MUST your hourly fee be? Or just take your yearly average time in harness and extrapolate.....
Of course this example is way too simplistic- I let Dude do my heavy work.

The point is, our time DOES have a cost. You have a human, aging body and mind that is subject to wear and tear. Call it life depreciation.
As for my personal practices: If I were sitting at home and got an emergency fill-in phone call, I may or may not do it ( if I'm alone, I may, if not, probably not

).
However, I would NOT book this sort of job in advance. Ever.
1) If you get referrals, and they talk to the client, they will be expecting the same rate.
2) My fees reflect the profit margin I need to meet my goals. If I can't meet those margins then my business is unsuccessful. Time to shut down and try something else.
Please keep in mind that my answer is based on that 30% reduction. I do have SOME room, just not that much.