i have tried constructing 90 degree it is not always 90 some times 89 or 91 is it important to be exactly 90 degree
In construction these marginal errors are allowed oftenly by the examiners as the accuracy of constructed angle depend on the factors that how fine the arc has been placed, whether the compass remained firm while drawing an arc, the sharpness of pencil etc.
While constructing an angle if you are keeping all the above mentioned factors in your mind then i am quite sure you will surely get an exact angle of 90°.
Moreover, try constructing the said angle several times, after some attempt you yourself find out the accuracy achieved in constructing different angles.
Hope you get it!!