I know I can right-click an address or select multiple addresses and "Send From", but that does not let me specify how much to take from each address to make up the tx amount.

Example:
Address x has 1 BTC and y has 0.2 BTC. I want to send 0.5 and at that opportunity wipe out address y to consolidate all my funds in x. So I want to make sure the transaction will be:
0.3 BTC from x + 0.2 BTC from y = 0.5 BTC to receiver
(y now empty)

Electrum only seems to allow me to specify the total tx amount, so it could end up being:
0.4 BTC from x + 0.1 BTC from y = 0.5 BTC to receiver
(y still holds 0.1)
... or any different variation with y still holding funds afterwards.

Is there any way to ensure the former?