Oh the magic of performance reviews.
Numbers based versus language based... Standardized forms versus individualized reviews.
The long and short of it is there is no magic bullet for this. If you want a numbers based one (and I say do what you can to make the boss happy) or you want a more nuanced review. Both have merit, a pure numbers one is great if numbers are all your organization cares about (see sales, profit, revenue, cost-cutting as examples). However such numeric systems fall short if you have customer service as a valued aspect of your organization or if you have decision making, leadership or innovation as a performance measure (Those were categories on my evaluation once) for obvious reasons.
You're best bet and its not good either is to create a hybrid solution. If you value your numbers... everything is graded on a 1-5 scale and you aggregate and average. So for true "perfromance metrics" like sales use the same 1-5 scale for those as you would for decision making skills... and then aggregate to an average score.
Right so, I sold $10,000,000 and my goal was $5,000,000... I should get a 5. However I have customer complaints in my file which say I suck as a service provider... I should get a 1 and therefore my average score is a 3 and I get bonuses and raises based on that.
Going further your best bet is to group employees by job function (not grade or title, but based on overall job function) and create ever so slightly personalized reviews. This way a file clerk doesn't get reviewed as if they had direct reports to manage or important decisions to make.
In the past I have actually had to review Processors and Data Entry folks with categories like "team leadership, innovation, and decision making" none of which applied at all to them... but they had to get a score because HR wouldn't sign off on my having completed their reviews without scores... there was no N/A box... it was also no surprise when NOT ONE of my direct reports gave a darn about what was in my review... they knew they were capped at 3% raises and garaunteed at least 1%... unless they were all zeroes which then they just expected to be fired.
Ultimately though whether you use letters or numbers, words, phrases, or one individual performance review form... if you do not honestly REVIEW PERFORMANCE it is all meaningless.
PM me if you want and I can give you some broad concepts to help you out with your scenario. However no truly effective quick fix exists.
In life, there is a lot less that could get better and a lot more that could get worse.
MBA Fin/MBS HR
My views only!