Electron beam fluence was measured at 200kV and 300kV by reproducing the condenser lens current settings used for parallel beam illumination in diffraction mode, in imaging mode, and illuminating the TEM's fluorescent screen at a magnification where the parallel beam filled the entire screen. The screen current reading from the microscope in nA was recorded and divided by both the charge on an electron (1.602 x 10 -19 C) and the size of the real-space illuminated area in Å 2 to give the flux density. This was repeated for a series of spot sizes, and yielded flux density estimates of 0.0252 e-/Å 2 /s for spot size 9 and 0.0147 e-/Å 2 /s for spot size 10 on the 200 kV Talos F200C. On this instrument, the screen current at the lowest flux setting, spot size 11, read as 0 nA, so the current was instead estimated by noting that the screen currents at spot sizes 10 and 9 were related by approximately a factor of two, and extrapolating that same trend to spot size 11. As another estimation, gain-corrected, flat field images of the parallel beam were collected on the Apollo detector, and the number of electrons measured per pixel was calculated from pixel values applying the known conversion factor for the Apollo of 16 counts per electron event . This yielded estimates of 0.01, 0.03 and 0.045 e-/Å 2 /s for spot sizes 11, 10 and 9, respectively. For the results and analyses shown here, we adopt the estimate from the Apollo detector for 200 kV data. On the Tecnai F30, where 300 kV measurements were collected, fluorescent screen current measurements yielded estimates of 0.17 e-/Å 2 /s for spot size 9, 0.10 e-/Å 2 /s for spot size 10, and 0.04 e-/Å 2 /s for spot size 11. Data in this report, unless otherwise stated, was collected using the spot size 11 setting at either 200 or 300 kV.