50/50 gives you -34* protection. You can go up to 70% AF/water but 50/50 gives you better heat transfer.
Other things I forgot to mention, take a good look at the batt cables, it is not uncommon for the neg clamp on ZJs to crack. I cut that clamp off and got a marine cable terminal, used an HD marine crimper to crimp it on the neg cable and bolted on a marine style clamp. So this way if I ever had to replace that clamp its easy. Also go back and check all the grounds, make sure that they are tight and clean. Make sure your serp belt is tight. Do not let the gas level get too low, fill it when you get to a quarter to cut down on condensation (like if its warm and damp out and then the temp drops quickly, condensation can happen in the tank and that can freeze). Keeping these things up I have never had problems getting a ZJ to start at close to zero.
You youngsters have it easy....engines with EFI that start by just turing the key....we old guys...had to deal with chokes....like this one on the pic of a Marine Quadrajet....that needed maintenance, adjustment and proper driver intelligence.....
2007 WK Limited Hemi QDII
1998 ZJ Laredo 4.0 Selectrac