V-Check/elastic breastplate

[welshiewhiskers]

I have a very wide cob who's saddle is xwide, its been fitted by a saddler and regularly checked but the saddle tends to slip back. I bought a v-check to help keep the saddle still but i am paranoid about it being too tight, the v-check is elastic with leather attachments to the saddle Ds and goes between the legs. For hacking out it fitted well but when i schooled today on the same setting it seemed really tight so i loosened it.
Can anyone advise on fitting it or how tight it should be?
Would a 5 point breastplate have been better?

any help would be very much appreciated.

Many thanks