Is there a standard conversion for calculating equivalent lumens in terms of wattage of different types of lighting, particularly Metal Halide, to that of LED light?
Not that I've seen. That's very hard because of the vast difference in directionality. LEDs are highly directional and the other bulk lighting methods like, HID or fluorescent, are omnidirectional or cylindrical in nature.
If you're using LEDs to replace one of those two for lighting in one direction, you'll need substantially less LED light to do it.
As an example with aquariums where the two aforementioned sources are being replaced in earnest with LEDs, the generally bright demand is met with approximately one Cree LED driven at ~700mA per 16sqin of surface area. This comes in at about 1/2 the wattage of the replaced sources. DIYers are seeing 7 month to <2 year paybacks.