No problem. Yes, on the GFE you can use a rough estimate. For example, if it's your bank's policy to require a 2-month cushion, put an estimate of that amount in Block 9 on the GFE. It's just a quick and dirty estimate.
On the HUD-1, the aggregate adjustment piece will be a negative number that ensures when the escrow account reaches its minimum over the year, it will not exceed the 2-month cushion. The aggregate adjustment and initial deposit are actually calculated using a worksheet.