I got my shiny Electrike at a chain of 26, so I was wondering how much of an increase it is in comparison to the classic 1/8192 chance shiny.

That's the formula. Let's do some math:

14747 - 40 (26) / 2621400 (41 - 26) 14747 - 1040 / 2621400 * 15 13707 / 39321000

Which is about 0.003486899.....

Or rounding it off, 0.0035%.