Starhawk, if you don't mind, here are a few more suggestions and associated rationale. I'm just playing around - feel free to take 'em or leave 'em.

Observation: the BOOST and 1S columns work best when they are adjoining; do this by breaking out vehicle as its own column (LV) followed by the flight number (FLT), looking something like this:
NET DATE LV FLT BOOSTR RTRN PAYLOAD(S) ORB MASS PAD
---------- -- --- ------ ---- ----------------------- --- ------ -----
2017-02-28 F9 031 32 EXPD Echostar 23 [11] GTO ~5500 LC39A
2017-03 F9 033 23-01 OCIS SES-10 [12] GTO 5300 LC39A
2017-04 F9 EXPD Inmarsat 5-F4 GTO 6100 LC39A
2017-04 F9 OCIS SES-11/Echostar 105 [15] GTO 5400 LC39A
2017-05 FH 041 XX-01 OCIS Falcon Heavy Demo Flight LC39A
25-02 RTLS XX-02 RTLS
2017-06 F9 RTLS Formosat-5 & [13] SSO 525+ LC4E
Sherpa SSO [14]
Notes:
– Ditching the parenthesis actually saves a couple of column spaces, and ends up more readable IMO.
– For FH flights, adopt the convention of listing the center core first, side boosters on the following line.
– The extra character on BOOST provides for future growth.
Cheers,
D