Equipment profile Fementer Top Up Water to make up for 10 gallon pot

Hello,
I have a 10 gallon Polarware pot and a 10 gallon rubbermaid cooler for the mash tun.  I would like to try my hand at 10 gallon batches, but accounting for losses such as trub, fermenter, boil off, results in too much volume for boil volume.  However, if I do have a 5 gallon el-cheapo pot.  If I assume I didn't, I could also put a value of 1.5 gallons in the fermenter/bottling  volumes and the boil volume goes down to 9.96 gallons.  Which would be great.  This 1.5 gallons of extra water can be extra run off from my mash too.  I did a mock with and without the top up water and the hop ounces do change, which leads me to believe that hop utilization would be accounted for.  Would this be an ok method?  Yes, No? 

I did try to search this topic and came up nil. Seems my search led me to "top up water for kettle"  which I have set to zero.

Thanks and take care!

I suppose I should just try it and report back.  I was wondering if anyone else would have a comment on this.  I think what I will do, after I have enough 5 gallon batches under my belt, is to use my polarware 10 gallon and my cheap 5 gallon pot.  I will fill the polarware up to a certain amount and then fill the rest in my 5 gallon pot.  I can boil in the 5 gallons to meet my target top-off value instead of using plain water.  The hops will be adjusted so that I get the proper IBU's in the 10 gallon pot.

Cheers!

Will be brewing 5.5 gallons of witbier tomorrow with my new setup.