DirtyHarry put a table together out there with info on how it is calculated:

I won't redo the formula here, but for those not subscribed to the page:
It can be difficult to figure out what squad level you need to hit a base's DFR. Linked below is a simple table to reference. Also, here's how the formula works:
"The maximum level difference is 15% of the attackers squad level. So the lowest base dfr a squad can attack is 'SquadLevel * 0.85'. The highest squad level that can attack a specific base dfr is 'BaseDFR / 0.85'
The special case here is for low levels. Due to rounding, there is a minimum level difference of 3. So a level 8 squad could hit a level 5 base." (This is why DFRs lower than 15 are grey on the chart - because the computation doesn't apply - it's the DFR + 3)
(Thanks to Micah for the details)