Unfortunately there doesn't seem to be a linear conversion between the two, even a rough one. For instance the planet/station I'm parked at right now has an orbital period of 362 days and is just over 1000Ls from the star, but Earth at 365 days is only ~500Ls from Sol.
It's still a useful guide though, if you see a planet with an orbital period of less than 100 days you know it'll be a short trip from the star; just be careful that you're checking the orbital period of the 'main' planet the station is around and not the 'moon' it may be around (because the moon will show you how long it takes to orbit that planet, not the main star).
Edit: Actually I've just noticed that the "Semi Major Axis" of this planet is about 2 AU (Earth is 1 AU), so there might be a linear relationship there; I'll do some testing in the morning.