Stem length
 

I have a new-to-me road bike and I've got a question: The saddle height and position I have set feels pretty good with the exception of that I still feel like I'm reaching for the brifters. Handlebar/brifter location is great when I stand but when I sit, I have to stretch 3-4 cm past my normal comfortable position to reach them or to ride my normal on-the-hoods hand position. I know that KOPS is a guideline and controversial at that but I'm wondering how you know if you should move the seat forward or get a shorter stem. I'm working on flexibility but have old back injury issues that may limit my ability to simply adapt to stretching out further.

Shorter stem; you've answered our own question, you feel 3-4cm (30-40mm) too stretched out; the only caveat to that I have is don't go 30mm shorter -- go 15 or 20 at MOST. Leave the saddle where it is.

I can't agree or disagree about not going 3 cm shorter. I do think it's important to make any stem changes in increments - small ones. A 1 cm change is quite large.

You put the brifters on a long ramp bend bar or something more designed to go with them?

if the reach gripping the straight upper part of the bar is good, that may be a factor.

Thanks for the advice. I have put a couple of hundred miles on the bike and finally decided to order an 80mm +/-7 degree stem to replace the stock 100 mm stem.

You can also get a shorter reach handlebar.
Norm in road bar reach is somewhere in the 85mm region
Many shallow cx bars have a reach of 75mm

I just swapped out the stock 115mm stem for a 100mm stem for the same reasons. I'm closer to the ideal position but I still think I can bring the bars back 5 to 10mm.

I got my 100mm stem from the LBS from the "$5 stems" box. I wish I had bought more lengths while I was there. :/