Hello Andy,
You’re right: a cap shroud tension of 90 kg is very slack! The tension on the cap shrouds should be more than 300 kg. The guidance is to set them at 20 – 25% of their breaking strain (which is about 1400 kg) with the backstay off. (Tensioning the backstay reduces cap shroud tension.)
To answer your question directly, the tension on the cap shrouds will be higher than that of the lowers. However, it will depend on the amount of pre-bend you want in the mast, which itself will depend on the cut/age of the mainsail. If your sail is getting a bit too full, more pre-bend will help to flatten it (before you use the backstay to flatten it more for de-powering).
It’s well worth spending the time to set the rig up, ideally using a tension gauge, which you’ve got (that’s the expensive part!). I recommend Ivar Dedekam’s book on the subject, which also considers sails, or, for just the rig tuning, there’s the Selden guide: http://www.seldenmast.com/page.cfm?id=6740 .
Anthony Russell
235/02 Sea Wyche
Anthony Russell
235/02 Sea Wyche