Mark, the number of shares you sell now are taken from the first...

Mark, the number of shares you sell now are taken from the first lot you bought, and any profit/loss is calculated from that first parcel.

You cannot elect to say you have sold this or that parcel.

If you bought 300, then 700, then 1000,
and now you want to sell 450.... 300 will be calculated from that first parcel, and the remaining 150 from the 700 parcel.

The next lot you sell will be calculated from the remaining 550 of the second parcel for starters.