I'm kinda stumped...

Part 1: How does ones accurately meter using a Neutral Density Filter and a (Sekonic 358) Meter.

Part 2: (Let me throw in a little twist here...) Using an off-camera flash, with sync speed @ 1/250

Thanks....

The filter is rated for a number of stops.  In theory, you can adjust by the number of stops to get the right value.

...my filter is a sliding variable filter from 1 to 3 stops.... there are no "stops" (Although I guess I could mark some...)

Position the filter they was you are going to use it.  Then put it in front of the meter and meter through it. Then take a reading and deduct one from the other and use that to adjust the setting for your camera.

Or... if you have a built in meter..just meter through it and check out the results.

What brand filter?  Doesn't it have markings on the ring?  I'm familiar with B+W filters but Tiffen, Singh Ray are marked as well (not sure about others).

You should be able to dial it in based on the markings.  If the manufacturer doesn't give you the stop loss for each marking, just test it at each mark and record the results.  So if it's a 2-8 stop filter, you know the start and stop positions and just need to calculate the exposure reduction value for the remaining points.

OP: First, as noted above, since your variable ND filter isn't marked with intermediate stops, you need to calculate your own by trial and error (and make little marks on the ring so you'll be able to get back to the same place, perhaps with a paint pen). That's the hard part.

The easy part -- with your Sekonic L-358 meter -- is to use the second ISO setting when you have the ND filter on. Example: Say that ISO 1 is at 200, shutter speed 1/250. Take a reading and determine your f-stop without the ND filter on. Say it's f/8. Now say you've got your filter set at 4 stops. Set your ISO #2 at four stops below ISO 200, i.e. 12.5 (your Sekonic will say 12, not 12.5). Then meter your flash with the ISO 2 button held down and set your aperture accordingly. In this case the meter should show f/2, 4 stops below f/8. Alternatively you can adjust your strobe's output or distance if you want to keep f/8 as your f-stop.

Thanks all.

My simple solution was to retract the dome on my Sekonic meter, and take a reading. The place the filter on top of the meter and re-meter. Simple.

At the low setting it is a 1-stop filter. At the high setting it is a 4-stop filter. With 18 markings between high and low setting, that means every 6 steps is 1 stop.

Whew!!!!

I would meter all the lights without the filter. Set your camera settings to be balanced for what you look.
The use an app like exposure calculator for android set the strength of your filter and the dial the numbers into your camera.

I do this all the time. Make sure you measure the ambient light, brightest, darkest and your strobe check that you are within the range of your camera.

First I would rely on the built in meter if at all possible but if you cannot what I would do is.
Take a meter reading with the light meter and camera with no  filter and make not of the two readings.
Put on the filter and set at the lowest ND setting and take another reading and if the light has changed at all also take a LM reading.
Set ND to what you want and take another TTL reading and compare to previous reading which will give you the Fstop correction.
Take meter reading with LM and apply correction .

Or bracket exposure, shoot RAW and hope for the best.