As Tussock mentioned, its really a guessing game unless you have the fancy OAL gauge (which I do have).
With a T3, I would just seat to fit the mag, as you probably can't get get near the rifling without single feeding.
Brycey, if you are still set on finding the exact distance. Get the modified Hornady 7mm-08 case ($10 from Reloaders or Serious Shooters I think) to fit on the end and you can use my OAL gauge and attachments.
I'm in St Heliers.
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