About the recalculation:
You can keep your set of random values from changing every time you make an adjustment, by adjusting the automatic recalculation
, to: manual recalculate
. (Re)calculations are then only done when you press F9. Or shift F9.
See this link (though for older excel version than the current 2013) for some info about it: https://support.office.com/en-us/article/Change-formula-recalculation-iteration-or-precision-73fc7dac-91cf-4d36-86e8-67124f6bcce4.