Another way to go.
In response to "Filing for a Lat/Long" in January 2012 edition of IFR: Would it not be simpler to use the OBS mode of the Garmin GNS530W to fly the PMM radial in the amended clearance?
You'd program the 530 for "... PMM MKG FAH...," and just before you reach PMM, hit the OBS key to suspend way-point sequencing and maintain PMM as the active waypoint. Then turn the OBS to 333 and off you go, flying the 333 radial from PMM.
When you get close to intersecting V510 (either by eyeballing it or using your second radio/OBS to tune to the MKG 296 radial), hit OBS again to resume sequencing. Then on the FPL page, select MKG -> FAH as the active segment (scroll down to FAH and hit direct-to key twice).
That's a bit of fiddling, but you have lots of time. There's no rush. And IMHO, it's a heck of a lot easier (and less error prone?) than trying to create a user waypoint based on a calculated Lat/Long. And you get to use that wonderful OBS mode that you know you always wanted to use!
We won't argue that your system will work. And if we got hit with this as a surprise in the air, it might be our solution. Simpler? That we might argue.
Simplicity or lack of it aside, being able to program the turn into the GPS has plenty of advantages: The autopilot can fly it, there's no eyeballing or extra attention required, and most of all, all the work can be done before you launch.
Another plus is you see the route with the turn on any moving maps, which can be a huge boon if you're also looking at weather and how it impacts your route.