If you are new, I understand your concern for accuracy. I started down that same path, but after a while, I just got good at guessing. The experienced eye works well for your average sized accounts, whatever size that may be. If youre stepping up to a significantly larger account, then yeah, be VERY careful.
Sure, you can figure square footage for mowing, linear footage for edging/trimming, but there ended up being many more factors than I cared to track for average accounts. Things like patio furniture; ornamental fences; garden hoses; the amount of low over-hanging trees; trampolines; cars in the driveway; customer relations maintenance (talk time); parking access; this list goes on and on and on.
I get a dollar figure in mind, and then try to take in the effect of supply and demand. So after accounting for as much as possible, I like to start high, then go into negotiations as needed. The accounts that end up being profitable are keepers. Everything else is subject to getting replaced by a new keeper because Im always looking.