General aviation pilot reported a safety concern with a note that appears on the Jacksonville sectional chart that obscures Class B airspace limits; which resulted in the reporter entering Class B airspace without clearance.

Narrative

Day VFR departure from SPG (Albert Whitted; Saint Petersburg; FL) enroute for 35-minute flight to private airstrip. The route chosen was a west departure with a climb planned to stay below Tampa Class B and then enroute; climb again over the Gulf of Mexico to overfly the SRQ Class C southbound along the shoreline. After departure; climbed to 2500 ft. to maintain an altitude below Tampa Class B - floor being 3000 ft. west of SPG. Consulted current Jacksonville sectional; displayed on EFB in-flight; to determine a suitable point for a southwest turn. Considering local traffic; ADS-B targets; chose to turn southwest to maintain safe clearance with traffic. Was also monitoring Tampa Approach on frequency. Turned southwest at a point that appeared to be under Class B with 3000 ft. floor. Maintained 2500 ft. until exiting what at the time was thought to be 3000-ft. floor Class B. Upon later review; learned that the area transitioned at 2500 ft. was in fact Class B with 1200-ft. floor. The confusion was caused by a note on the sectional chart that nearly completely obscures the delineation between 3000-ft. floor Class B and 1200-ft. floor Class B. Please review the current Jacksonville sectional chart for the referenced delineation line that apparently extends westward from a point south-southwest of SPG. The obscuring note reads 'See NOTAMS/Supplement for Class D eff hrs.' I believe that in-flight I mentally connected the Class B 1200-ft. floor delineation line with the double-stripe highway line; I-275; that extends southward from the overlaying note. When at SPG (Albert Whitted); I rarely fly westbound. I sensed that something did not feel quite right about the route and altitude but upon an in-flight follow-up glance at the chart convinced myself that I was flying under Class B with 3000-ft. floor - thus 2500 ft. altitude. Someone less familiar with the airspace in this area could easily make the same erroneous observation. Suggest that the sectional chart be revised so that there is not a note that obscures the east-west 3000-ft. floor to 1200-ft. floor delineation line; that is; so that the line is plainly visible along its entire length.