I've always used a roughly similar technique and have never seen the need to drain rice after cooking. Great with normal basmati, but I am having trouble getting it to work as well these days as I have taken to using a 50/50 brown / white rice mix. Any suggestions?
For a long time I measured both rice and water by volume. I used a particular little scoop that was just the right size for a portion of rice, then I'd use two and a half scoops of water. I found that different batches of rice could need different amounts of water, so, each time I'd start a new 5 kilo sack of basmati I might need to tweak the volume of water up or down. Once the ratio had been found it worked every time for that batch of rice.
Try measuring until you find a formula that works for your 50/50 mix?
I think how much water you need depends somewhat on how you cook your rice. I bring to the boil, then turn it right down to the merest of flames, and keep the lid on all the time. That means almost no evaporation so I don't need much water. If you give it a bit more welly on the gas, or like lid off, then you'll obviously need to add more.
Nowadays I just measure by eye, chucking in rice and water so it looks about right. I'll then have a quick peek two or three minutes before end of cooking time, if it looks either a bit wet or dry I turn up the heat or add another splash of water, as required.