I'm building a multiband ladder line fed doublet for field day and I'd like to cut my wires as close as possible the first time to minimize trips up and down for tuning the antenna...
I think you are missing the point here.
If you are using ladder line feed with a tuner in the shack, the exact length of the antenna
doesn't matter. The antenna will work about the same on the fundamental with +/- 20% or
more variation. The only reason you have to tune an antenna is to minimize the SWR on
your feedline when you aren't using a tuner, and with ladder line you'll have a high SWR
and need a tuner anyway.
I did just what you are proposing last Field Day: after considering the weather (rain) and
resources available (I was on crutches) I threw together a 40m doublet with ladder line.
I just estimated the length as somewhere between 30' and 40' each side based on the length
of my arms (but making sure both sides were the same length), tossed it up in a tree, tied
off the ends, threw it on the tuner and started working stations.
Don't make it more difficult than you need to!