Do I need Backstays on my Mizzen?

[don1]

Hi,
Can anyone help me with my question.

I have a 1973 Gulfstar 44 Ketch rigged MS,and am wondering whether I need backstays on my Mizzen, which has swept back spreaders.
The mizzen is 32 ft high, and the upper shrouds are 18" behind the centre of the mizzen mast.

I was wondering whether there is a formula for the angle of the spreaders to determine whether a backstay is needed?

If anyone can give me any help with this it would be much appreciated.

Thanks