Hi Linda, congratulations on your double row strand, for a first attempt you have done a fantastic job.
When using new pearls you can finish off the first strand as long as you know the starting off length (including the clasp).
If the necklace was going to be a graduated necklace, recommend work out the graduation first on a board or using a temporary thead\line, and then as above.
Now if this was a restring, DO NOT finish off the first row, as variations in technique and thread used can affect the lengths, and how it sits.