You calculation looks complicated, it's not required. As for the 3.9W 3dmark Temash we have an official score from AMD:
Test conducted in AMD Performance Labs using FutureMark 3DMark for Windows 8 Ice Storm Test as a metric for GPU performance. The AMD A4-1200 APU-based system scored 11,920 while the Intel Atom Z2760 based system scored 2431. The platforms tested were the “Larne” reference platform with an AMD A4-1200 Dual Core 1.0GHz APU, AMD Radeon™ HD 8180 series graphics, 2GB DDR3-1066 system memory and Microsoft Windows 8, versus an Acer ICONIA W510-1666 with an Intel Atom Z2760 1.5GHz CPU, Intel integrated graphics, 2GB DDR3-1066 system memory and Microsoft Windows 8. TEM-18
In 3dmark Ice Storm Z3770 Bay Trail-T seems to match almost the 3.9W Temash score. However I think the much better CPU in Bay Trail-T helps a bit in 3dmark Ice Storm.