Refractive correction is never measured in anything except 0.25 increments... therefore your -7.8 would unfortunately be rounded to -8.0 which puts you on the very edge of the limit.
No two refractive exams are ever the same therefore there is a chance that if two people did your eye test on same day they would have an error factor of 0.25 to 0.50 diopters so you could either be measured as having -7.75 or -8.25 depending on who did it, your tiredness, eye strain, lighting etc etc.. number of factors.