the general rule of thumb is to use 80% of the weight of the liquid malt (DME = 0.8*LME)
I'd run it through brewing software to double check the math and adjust as needed. Check out the http://hbd.org/recipator/ It has a free spreadsheet that will generate beer stats and I've found it works fairly well (limited hop and adjunct selection but that won't matter for what your looking at)