Sorry for being pedantic in the following, but there are a lot of confusing things in your statement and other statements above.
The difference between 1/250 and 1/200 changes cannot change the amount of fill. The flash dumps its light in 1/1000 or faster. The amount of light supplied by the flash doesn't change between 1/250 and 1/200.
What does happen is that the shutter speed change allows more ambient light by 1/3 stop. This means that for the same flash output the fill to ambient ratio by decreases by 1/3 stop. This becomes a problem if the flash was at full power with an ambient exposure at 1/250.
Gary in SE Michigan, USA. Nikonians membership - My most important photographic investment, after the camera. D4, D810, D300 (720nm IR conversion), D90, F6, FM3a (black), FM2n (chrome) YashicaMat 124, Graflex Speed Graphic 4x5 My Nikonians Gallery & Our Chapter Gallery