I've never been able to get a reliable flowrate using gravity - or, for that matter, using a 1/4" line tapped on the sump return line. I actually blew apart a reactor once because I had the feed come off the sump return line and had a needle valve on the output of the reactor instead of the input. I thought "no way can 4' of head pressure be a problem" guess I was wrong.
And having the needle valve on the input drove me crazy because I'd set it to one thing, check it a couple of hours later and it would be something completely different.
The most reliable method I've found so far, out of the ghetto methods that is, is to have the needle valve on the output, and a small powerhead on the input line. The powerhead just pressures up the line ever so slightly to promote a steady flowrate, but can't push enough pressure to create a problem in the reactor.
A variation on this theme is to use an Aqualifter pump. Looks like an air pump but is used on water. Again, just pressurizes the lines enough to push flow but not enough to create a pressure problem (leaking flanges or broken seams).
The best way IMO is to use a metering pump. But some reactors create a siphon so that impacts your choice of pump - a piston pump won't stop a siphon. If you have a siphon your best bet is probably a peristaltic pump. The Aquamedic dosing pumps are inexpensive but fixed flowrate. So for about $100 you can have a guaranteed 50ml/min flowrate.
If you want to go an extra step and have adjustability, there are lab grade peristaltic pumps that offer a variable flowrate. Basically the motors are DC so you vary the voltage to vary the speed of the pump head rotating. Plus there are different pump heads that can be swapped out that give you different flowrate ranges (ie, smaller tubing versus larger tubing, or diameter of the pump head itself - ie. the more distance the rollers travel per rotation - the more volume dispensed per rotation). Problem is these are a little bit spendy. Retail on these suckers new is like $1000 but you can find deals on ebay. Here's an example:
http://cgi.ebay.com.sg/Masterflex-75...mZ200162085812
(You have to search on something like "peristaltic" or "Masterflex 7520" every week or so. This one is a good example of a good deal on a pump, but they charged way too much for shipping - but I've seen these pumps on average go for around $80-100 and usually around $40 for shipping - I guess they're heavy and bulky).