I combine PM with late-addition extract. I look to collect 3 gallons of wort from the partial mash, to which the hops are added. Boil down to 2.5 gallons, add extract, and top off to 5.5 gallons in the primary.
Here's an example:
Black Dog Ale (Robust Porter)
3# 2-row Pale Ale malt
0.5# 60L crystal malt
0.75# chocolate malt
0.25# black patent malt
This partial mash provides me with 3 gallons of wort at 1.037. Boiled down to 2.5 gallons, I have a wort of 1.044.
If I were to top that off to 5.5 gallons, the OG would be 1.020. But the late addition of 4# of Briess Organic Light DME brings the 5.5 gallon OG to 1.051.
Be sure to check your IBUs carefully. The above recipe includes the following hops schedule:
1.5 oz Willamette 5%AA - 60
0.5 oz ditto - 20
0.5 oz ditto - flameout
With the gravity provided by the partial mash alone, the IBUs are 39. With the DME included, ProMash says 30 IBU. Since the wort being boiled doesn't have the DME, the first number is correct.
In summary, I boil a partial brew length with fermentables provided by the partial mash. So it all balances out!
Cheers,
Bob