What cut of ground beef is ideal to mix with ground lamb for kofta? I'm attempting to replicate kofta that I had at a Mediterranean place last week. They were a mix of beef and lamb, yet neither flavor was more pronounced than the other, in fact it had a sort of flavor all its own. It wasn't at all fatty, yet it had an amazing seared crust on it. 

I've tried, in vein, to replicate this recipe (which apparently they keep as a closely guarded secret). I've got the spices right, at least I think, the usual cinnamon, cumin, clove, etc that you'd find in kofta. To be clear, I can make kofta, getting the lamb and beef proportioned seems to be where I'm failing.

Where I think I'm going wrong is the fat content of the beef. I've tried chuck and round, and the results were kofta that tasted completely like lamb (round) and completely like beef (chuck). I'm using a 50/50 mix. 

When mixing ground with lamb, what cut should I be using that has the best shot of balancing? Should I be going heavier on the beef or lamb in the ratio? I can keep going with trial and error, but after a lot of error, I thought it wise to look for a little help :)
Try half and half sirloin and chuck for your beef. If you're grinding your own or can otherwise get quite specific, use sirloin tips. Ground, they're just as good as the much more expensive top sirloin. Sirloin will be low in fat, but will still have great flavor. I have used a combination of 1/2 lamb, 1/4 beef chuck and 1/4 beef sirloin tips to great success in Gyros meat (the meatloaf type).
Avoid round. There's a reason that round is often sold as generic stew-meat. It's a cheesy way for stores to sell what is often not very good meat. It's good for a few specific purposes, this isn't one of them.
EDIT: I just learned something I didn't know. Since I don't know what terminology is used in the Philippines, I bring you this handy diagram from Wiki.

I use the American terminology for sirloin, top sirloin, sirloin tip, round and chuck.
From Wiki:

The sirloin steak is a steak cut from the back of the animal.
In U.S. butchery, the steak is cut from the rear back portion of the animal, continuing off the short loin from which T-bone, porterhouse, and club steaks are cut.The sirloin is actually divided into several types of steak. The top sirloin is the most prized of these and is specifically marked for sale under that name. The bottom sirloin, which is less tender and much larger, is typically marked for sale simply as &quot;sirloin steak.&quot; The bottom sirloin in turn connects to the sirloin tip roast.