If the reticle subtends properly at 27x and because of poor light conditions or mirage you cannot really use max power on the scope, then you can't use the reticle (without some math on the fly) to range or hold.
Much safer to have the reticle subtend properly at less than max power on such a high power SFP scope.
I rarely use my Minox ZP5 5-25 or Cronus 4.5-29 at max power, generally because I want to take advantage of the increased FOV at a lower power, even when mirage is low to nonexistent. Of course this is why I prefer FFP scopes; I pick the power that suits my needs, accommodating mirage, low light conditions (rarely a problem with the Cronus or the Minox), desired increased field of view, etc. and the reticles always subtend.