I would also go and speak to the vets about it, in general sponges go in for 12 days and it is best for the ewes to be in the field next to the tup. Then sponges come out (wear disposable gloves, quite a bit of gunk comes with the sponge) and then tup goes in - all excited, we didn't manage to put raddle paint onto the chest of ours...
- and he will get straight down (or up) to business. Change raddle colour 2 weeks afterwards, and see if any are repeated on on day 17 or thereabouts.
It is probably best if you get someone to show you how to insert the sponges, especially if you have any first-timers.
In terms of time count back 145 days from you preferred (middle of) lambing date, then add on 14 days. Only problem is if/when he repeats on one or two - but at least you will know what/when to expect lambs.