This addresses an omission from Part1, showing how to adjust the zoom factor from 1.0 according to how far away you sit from your monitor. This gets slightly more mathematical than Part 1 but you don't need to understand it, you just need to plug your own numbers into the formula to calculate the zoom factor. (To get the 'arctan' function on Windows 7 calculator you need to press the 'Inv' button and then the 'tan-1' button!) ... https://www.youtube.com/watch?v=ikwToOzX_UA

In 1927 Charles Lindbergh put Claude Ryan on the map by flying non-stop from New York to Paris in an aircraft with his name written on the tail. Although the 'Spirit of St. Louis' was built by Ryan Airlines, Ryan had already sold his stake in the company and left. But he walked away with the brand and in 1931 he began work on a new aircraft, the ST, which in 1934 would become the first product of the Ryan Aeronautical Corporation. The ST was a low-wing monoplane that used Al Menasco's B.4 'Pirate' engine, and improvements in the Menasco engines quickly established the ST, and its successors the STA and the STA Special, as formidable competition aircraft. The STA Special also attracted interest from the Mexican Air Force and before long Ryan was taking orders for a militarised 'ST-M' version from Mexico, Honduras, Guatemala, Ecuador and Nicaragua. Production continued into the early 1940s, and ST-Ms also sold to China and the Dutch East Indies. The US Army Air Corps was also interested and in 1939 it bought an STA for evaluation as a primary trainer, calling it the XPT-16. This was followed by orders for a further 15 as YPT-16s and 30 as PT-20s. Engine experiments throughout 1940 settled on the Kinner R-440 as the engine of choice, and the YPT-16s and PT-20s were all subsequently retrofitted with Kinners. From here on development diverged from the ST-M as Ryan redesigned the STA's elliptical fuselage into the rounder, longer cigar-shape of the ST-3. The Army bought 100 and called them PT-21s, while the Navy bought another 100 and called them NR-1s. The definitive version appeared in 1941, with the Kinner engine upgraded to the 160hp R-540. This was the ST-3KR, or PT-22 'Recruit', of which the Army ordered 1023. ... https://www.youtube.com/watch?v=5Ubm_3DNc5c