Try 126.5021066. This should be the exact perimeter.

The formula I used:

where is the eccentricity ( in your case), and is the semi-major axis--53/2 in your case.

I doublechecked by plugging in your semi-major and minor axes (a=53/2 and b=25/2 respectively) into Ramanujan's formula:

which gives 126.502111621. A very close approximation.

When in doubt, and when you don't want to deal with calculus, go with Ramanujan's formula. Except for very flat ellipses it will give you a circumference that is exact to four or five or more decimal places.

I had to look up "caliper" on wikepedia. Are you plotting out a garden?