C99 pilot reported the aircraft rolled forward unnoticed after engine start after they took their feet off the brakes while attempting to refasten the seatbelt which had become unlatched.

Narrative

I departed late and last after deferring loading due to a broken nosewheel turning stop discovered during pre-flight; so I was trying to work quickly. I did not verify that my seat belt was well-secured by pulling on it when I fastened it. It appeared secure and I started my engines. When I reached over to change a frequency to contact tower for taxi clearance (operating as ground/tower combined) the buckle came apart and fell to my side; so I had to put it back on. I moved my leg to get to it and took my feet off the brakes looking down to my side to find it.The parking brake handle was pulled-set; but I later learned that it had about another 1.5cm of travel that was much firmer to pull than the first 1.5 inches of travel which would have actually set the brakes. The aircraft started rolling forward while I was heads-down and made it about a full length of the aircraft (40-50 ft) before I realized it was moving and stopped it. There was no damage or further incident. I guarded the brakes closely and re-fastened my seatbelt verifying positive latching and then continued with normal operations.In my peripheral vision; I noticed the movement outside the aircraft after about 30 feet of forward travel. A combination of my failure to apply significant force to the seatbelt after latching to verify it was secure; a lack of familiarity with the parking brake handle in that particular aircraft leading me to believe it had been properly set by feel; and not looking outside the aircraft for a moment before looking for the seatbelt to verify that after taking my feet off of the brakes that the aircraft was remaining stationary. I applied brakes firmly and immediately.Suggestions: Applying significant force to the seatbelt after latching to verify it is secure. Becoming familiar with the feel/personality of the parking brake handle in the particular aircraft; this could be accomplished by intentionally testing the parking brake prior to relying on it for momentary disengagement of feet from the brake pedals. Making it a best practice to look outside the aircraft for a moment any time feet are disengaged from the brake pedals to confirm there is no aircraft movement especially when there will also be focus on something inside the cabin while feet are off the brakes and limiting the time with feet off of the brakes to when there is no other reasonable choice but to do so.