Transmission and propagation
You're generally thinking along the right lines, but you're oversimplifying in some areas.

For example, with stop-and-wait, the transmitter can't send a second packet until the the first packet has been fully received, and the ACK has propagated from the receiver back to the transmitter. Note also that "piggy backing" implies that the ACK itself is part of a full-size reverse channel packet that must be fully received by the transmitter.

This round-trip delay also factors into your sliding window calculation.

Also, double-check your math while you're at it.