Hello,
If your lighting unit is dimmable then you have nothing to fear - as long as you dim the light. Of course, a PAR meter is worth it's weight in gold, but it dose cost a lot of gold to get one, which is a real shame. If you can find google reports of others who review the PAR values of your particular unit then you can get a clue. When the tank is first flooded a good lighting policy is to limit the PAR to about 40-60 micromoles at the substrate level for the first 6-8 weeks or so. Then the intensity can be increase gradually to increase growth rates if desired.
Cheers,