[Drunken F00l]

You can set the overall minimum to 70%, but there is no built-in way to make a minimum on a per-map basis. You're free to edit the lua script to make it do whatever you like, however.

Something like this is an ugly hack, but will do:
Under:

Code:

local defpercent = convar.GetInteger("DF_mapveto_defaultpercent")

Add:

Code:

if(nextmap == "cp_MapYouHate") then
    defpercent = 70
end