Thanking you very much - in advance --for helping me with these questions
Rashi DH Mipnei she'Mafsid writes that the Techum on each side of the city is 1428
Amos from the city. Where did he get the extra 28 Amos from?
First, from Yeshiva Shaare Chaim
Take a look at the attached picture (also viewable at www.dafyomi.co.il/eruvin/discuss/eruv-056.qa6.jpg). Rashi's intention is to calculate the length of the Techum of a city (in a direction perpendicular to the city's border), that is, the length of lines AB & AD, had we measured the Techum of a city using a diagonal line extending 2000 Amos from the corner of the city (that is, AC = 2000 Amos).
Since AC is the diagonal of square ABCD, and Chazal calculate the length of the diagonal of a square as 7/5 times the length of its sides, then AC = 7/5 of AB, or AB = 5/7 of AC.
5/7 of 2000 Amos (that is, AC), is 1428.57 Amos. That is why Rashi mentions the figure of 1428. (As for why Rashi does not mention the remainder, that is discussed by the Maharshal and Maharsha.)
Best wishes,
M. KORNFELD
Kollel Iyun Hadaf