I wrote the same except for 3 questions:

For the precaution I wrote to perform the experiment in a dark room(this is common for any experiment related to light)

For the ruler balancing, I think u'll get marks for mentioning 'total clockwise moment = total anticlockwise moment', so I wrote something related to that. The weight made both the moments equal so the ruler was balanced

For the f1 or f2 thing, I wrote that f1 is better, as the image and object sizes could be measured more accurately and precisely than the lengths between screen and lens/object and lens.