The Equipment Rules mandate that your VHF be of a minimum wattage (25W) and that the connection to your masthead antenna not lose more than a stated amount of that wattage due to cable loss, dirty connectors, or other factors.
Some have found that calculation difficult. There are numerous online resources to calculate cable loss, and Dustin has kindly created a spreadsheet to help you with that. It's located here. That's a Google Sheets page, you can also use the spreadsheet attached.
NOTE that measurements are in meters. Further, this presumes that your connectors are clean, dry, and corrosion-free.