certain amount of height above the board- certain height (the rest seems unnecessary to me, but 'amount of' is wrong)
( the width of the circular part is one third ofthe radiusofthe tempered glass lid) - as this is a new sentence, the brackets don't make sense to me
kept away from being struck- from the ball dropped
whose included angle are 90į- not sure, but I'd say which are at 90 degrees
joining lineof the- line joining the...
PS what do you mean by the 'fanning part'? It could be right, but I don't understand the term
Student or Learner