According to my vet, and this is what I did last month when my silkie girl had her first heat. 25 days from first bleeding, special care that they DO NOT get in contact with each other from day 10-16, when her discharge will be a pinkish color.. supposely those are the MOST fertile days..
I send my boy to my mom for 22 days and still kept a baby onesie on him when he came back home at day 23 to day 25.
Best of luck..
hugs,
Carmen in nj