If your needle valve doesn't provide enough adjustment, you might try to find a Clippard MNV4k2. If there is a local distributor you can get it for about $11. By mail order from fish stores, about $18. It isn't wonderful but a lot of people including me use it successfully. Also with the right adapters you can connect it directly to your regulator (after you remove the huge needle that is on there now).
For more info on all kinds of available needle and metering valves, take a look at my web site: