Overshot pre boil volume

Brewer-dad

I way overshot my pre boil volume, which I attribute in part to some changes in my practices after realizing I was leaving wort behind in my mash tun. So now I have a good amount of leftovers after sparging, which I have boiling away next to my main batch. I took 1.5 gal of the excess(definitely at least two probably almost three gal left but don’t have a bigger pot). My thought is to boil it down a good amount to reduce and then add at flameout.
I missed my pre boil gravity by nine points so I’m hoping between this and a longer boil I can make up some ground. Any thoughts on reducing leftover runnings like this? While I would like to maximize efficiency where possible I’m not obsessed with it, so if the consensus is this is a bad idea I’ll just toss the extra. I’m doing the Winter Warmer from The Homebrewer’s Almanac.
I apologize if this post is disjointed juggling the boil and kids unexpectedly.

DBhomebrew

Boil off that extra wort as much as you want. All the way down to a thick syrup if you can. Skotrat's Traquair clone and this forum's Gunstock Ale both use this as a feature.

TheMadKing

That should work fine as long as you acidified your sparge water.
If not, you might have oversparged and pulled out tannins and end up with an astringent beer.

KeizerBrewr

To answer your question, yes, that would work.
Are you measuring out your total recipe volume and using the amount up? I have had a couple batches that I goofed and didn't pay attention to what I needed and just shot from the hip when it came to sparge and left a lot behind. I get leaving a bit behind from tun and kettle losses, but that sounds like quite a bit.

Brewer-dad

Thanks for all the replies! I went ahead with it and ended up .02 low, which I am perfectly happy with.
@TheMadKing to this point I have only been acidifying at mash, maybe I will adjust and do both. Hopefully all the other things going on in this recipe help counterbalance any astringency I added.
@KeizerBrewr I followed the recipe which gave the mash and sparge volumes. But I do agree with you it seems like a ridiculous overshot. I am batch sparging in a cooler, and a few batches ago switched from a bazooka to a BIAB. This time I actually did the whole bag lift thing so left almost nothing behind. I had about 4.75 gal go into my kettle out of my 5 gal of mash water.
This is probably my fifth AG batch, and over that short time I have changed a number of things. Obviously I have some dialing in to do! Thanks again!