Just in case the rental doesn't work out you can use a plan "B".
I could not locate one to rent or use so here is what I did. Ironically I used this home made tool on my rear rims and got it right. I didn't use it on my front rims and ordered the wrong offset. It does work.
1. Make sure the wheel is removed and the car is on jack stands simulating ride height.
2. Take a piece of 1/2" angle (steel or aluminum) that is about 4" longer than half the outer diameter of the tire you are going to use. Example: 26" tall tire you would use 13" + 4" = 17".
3. One inch from the end of the angle, drill a 1/4" hole.
4. Measure 13" (in the case of a 26" tall tire) from the drilled hole and mark it. This will be the mark you want to line up with the center of your axle.
5. Clamp the angle to the face of the axle making sure the centerline mark lines up with the center of the axle.
Hint. I ran the angle between two studs making sure to cross the centerline of the axle. Then put a large washer on each of the two studs. The washers went over the top of some of the flat on the angle and when I tightened the lug nuts, held the angle tight against the axle surface.
6. Take a piece of 1/4" all thread say 12" long and thread one nut onto it maybe 3".
7. Insert the all thread into the angle and thread another nut onto the side sticking through the angle.
8. Move the all thread as far outward as you can until you hit the outer fender well.
9. Move it away from the fenderwell 1/2" and tighten both nuts.
10 Rotate the axle 360 degrees looking at the end of the all thread for clearance.
11. Once complete do the same to the inner fender well.
This will give you the distance from axle face to inner and outer fenderwell, which will tell you the maximum tire width you can run. From there just do the math to determine offset.
Tips:
1. Move the axle as far up in the wheel well as it will go (suspension bottomed out) and as far out of the wheel well as it will go (suspension extended) and make a check as well.
2. Many axles have a raised portion in the center that stands away from where the rim surface is. Just be sure to consider this when calculating.
Best of luck.
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