If your value is in G1
, =(ROUNDDOWN((G1+2000)/5000,0)+1)*5000
should work
This takes the value in G1
and adds your 2000 mb buffer in order to determine how many 5000 mb "blocks" this space would require. The formula then rounds that number down to a whole number of "blocks" (which is then too small for the current value + buffer) and then adds an additional "block" to allow enough space for the current value + buffer. Then multiplying by 5000 gives you the total number of mb.