So what's the over/short that you'll just get a fourth disparate reading?
At least with this 9-12 range, you know you don't want to change the amount at all. If it's 9, you don't want to lessen your dose and if it's 12, you don't want to increase your dose. Ergo, don't rock the boat.. whatever you're doing continue to do so until you have an idea what the number really is.
The Salifert should be giving you a reading in meq/l right? I assume when you say 12.5 it really told you a number like 4.46 which is probably somewhere in the vicinity on 0.05 on the syringe?