Another Speedo Gear Question

Sorry, I have an automatic, all of the guides I've been able to find indicate that there isn't a speedo gear that will correctly compensate for 4.10's, the 23 tooth gear I had put in is supposed to be the closest. Using one of those gear calculators, it came out showing I needed a 25+ tooth gear...which they don't make. I'm ok with the speedometer off, I just need to figure out what meant by the "closest". Does that mean 5% off, 10% off, 15% off??? I'm currently in the 10% off range at 70 MPH (true speed). Also, I"m trying to determine if the 10% off is consistent throughout the speedometer range, by that I mean when my speedometer shows 40 MPH, does that mean I'm really doing 36 MPH. I just need some kind of guideline to mentally compensate. I don't want to be driving 25 in a 35 and not know it!!

Thanks for the input, I found a formula somewhere and I think my speedometer variance is 15%, the speedo gear I used should correct for the change from 3.27 gears to 3.55, leaving the unadjusted variance the difference between 3.55 and the 4.10's I installed. I think I'll have to breakdown and get a Speedcal, I do enough math at work!