The grade 2 paper that I use (Freestyle's Arista) I usually develop for closer to 2-3 minutes; inadequate development can sometimes cause you to miss shadow details.
In the past I've rated this paper around EI=3; but doing more controlled development tests this summer revealed that with the developer mixed fresh, and its temperature maintained close to 68f, I found EI=12 to be pretty good. FYI I've used both Neutol WA and Ilford's Universal liquid paper developer; both I dilute around 1+15.
Regarding the preflashing, my setup uses a type S11 bulb, which is a white frosted round globe, about the size of a golfball, in standard household socket base, built into a soup can housing with a 3mm exit hole; the lamp is suspended about 30 inches above the paper when preflashing. I've found 10 seconds a good preflash time with my light, paper choice, EI rating and developer methodology.
For your setup, you'd want to get a dim enough light source such that the preflash exposure times can be accurately timed; I time mine by a standard Graylab darkroom timer that doesn't permit sub-1 second exposures, so getting the light intensity low enough to permit exposure times in the 5-10 second range is important for accuracy.
I know you don't want to build your own light source. You could try a light dimmer to reduce the intensity; but the problem is that this changes the color temperature of the lamp with changes in voltage. You end up chasing your tail with calibration tests using a light dimmer, since the spectal response of the paper is not linear with wavelength, trust me! Spend a bit of time to make a reliable light source, then do some calibration tests to get your prefered preflash time down. I prefer to preflash such that an otherwise unexposed negative, when developed, yields a faint light-gray tone.
The idea with a paper negative is to fit all of the scene's brightness range onto the limited tones of the paper. So you don't want a paper negative to have the wide contrast range of a finished print; you want the tonal scale compressed, but maintaining detail in both shadows and highlights (ideally; sometimes this isn't possible, in which case you pick which is more important). Then, when you contact print (or scan and tweak in PS), you can extract the scene's full tonal range into the finished print.