A lighter gravity wort will suck the acids out of whatever handful of hops faster than a higher gravity wort can. The math sucks.
Typically extract brewers (Been there done that) will (eventually as they climb the food chain) do all the hop aditions as directed in the recipe they are following, and then add the extract after they turn off the fire under the kettle. Then you can stir in the extract (a whisk works great for this), put a lid on the kettle and let it Pasteurize a little bit just in case, then chill and then pitch the yeast.
This is where partial mash brewing gets to be the worst of both worlds, especially with some Pilsner malt in the mash tun. I have had, actually, zero trouble with partial mash ales.
With any recipe using Pilsner malts you want a good vigorous boil - lots of head space in the kettle - and you have to chill them promptly. So when partial mashing with lager malts, you end up between a rock and a hard place at the end of the boil.
If you put the extract in before the boil is over to kill any nasties that might be in the extract, your late addition hop utilization is going to suffer. If you put the extract in after the boil you are taking the chance that your yeast will outcompete any microbes that happen to be in the extract.
I don't have any brewing software but I am pretty sure at least one of the big names will let you change the gravity of your boil as you add the extract and re-calculate the late hop amounts for you.
Sticking with ales in either extract or PM, I have done fine adding the hops to my low gravity boils and adding the extract at flameout.
HTH
P