Hopefully my last question for this spreadsheet!!

I have a spreadsheet that calculates the effect of weight on a balance wherever the weight is placed.
I now want to be able to place some weights where I want them rather than have the spreadsheet calculate all the options for me.
To explain say there are 4 weights a,b,c,d and 4 positions 1,2,3,4
I can calculate all the permutations for the effect of these weights in all positions and return the optimum balance for me BUT i want weight "a" to be in position 1.(assuming position 1 address is a2) and return the optimum balance with this given condition.
I guess for a function I can use =if(a2=1,a2*ref arm,a2*999999) a2*999999 would return an unfavourable result andtherefore not be selected as the optimum.
My question is how can i create the input form for the user to stipulate which weight should be where so it is user friendly?
cheers
Nobby