Mark, half the fun of blending, especially if you are trying to match something, is experimenting. I would start with bowl sized mixtures. I make a ring the size of a half dollar and put some of one in it to match the size, and then a dime sized circle to spread some latakia in. Then increase and decrease the circles.
As a die hard McClelland fan, I am embarrassed to say that I have never tried 3 Oaks. So, I have no idea whether you should start with a tiny bit or move to a 50/50 mix. The circles will help keep up with whether you added 20%, 30%, or 50% by estimation.
Does it have any Turkish in it? Sometimes they add just a little of one of the many orientals to play off the latakia. Sometimes with some blends, they can have a massive amount of latakia in it, but because of the play of blended tobaccos it doesn't taste like a lat-bomb. Russ posted about this when he first sent out samples of WhiteKnight.
But, keep an open mind while you are experimenting. You may find a blend that you make to be even better than the blend that you are trying to match. I have just randomly mixed some stuff haphazardly, and made something I couldn't reproduce, but I had wow'd myself with. For those of us whole like to smoke for new flavors and experiences, besides aging blends or baking tins, blending is a great way to try new things that no one has ever tried before.