The 300 f4 IS has a filter thread of 77mm, so a step down adapter would probably be needed. I do not own a ring flash.
Distance to front element was approx 14", I will set up and measure with a tape later and add a more accurate number. This was shot in the field, or more accurate in the four acre Meadow which is part of the SC Botanical Gardens in Clemson, SC. An area that holds many photographic fascinations for me.
300 f4.0 IS + 500D, f11, 1/320, ISO 400, IS off, Tripod used, camera 5D.
Here is the full frame as exported by Lightroom (another subject, but I am enthusiastic for LR since spending some time to become somewhat familiar with it). The white part of the bud is around 3/8" across. Default processing by LR was a lot brighter, I brought it down for esthetic reasons, whites are not blown out.