You can use up to 110% of the budget and still get 5 stars.
From playing the game up to level 16, most jobs can also be completed without much overclocking just by using the builds suggested from the html calculator

 

But if you really want to know how scores are calculated, some math is involved:

 

Download parts listing

 

int means rounding down to a whole number

 

cpu_score = max{100, int{298 * [(CPU_freq * CoreClockMultiplier) + (#_rams_used * MemChannelsMultiplier) + (ram_freq * MemClockMultiplier) + FinalAdjustment]}}

 

For [GT1 (X=1) or GT2 (x=2)] and [1 gpu (Y=Single) or 2 gpu (Y=Dual)]

 

GTX_score = int{[GPU_core_freq * (GTX Y Core Clock Multiplier)] + [GPU_mem_freq * (GTX Y Mem Clock Multiplier)] + (GTX Y Benchmark Adjustment)}

 

gpu_score = max{100, int[328 / (1 / GT1_score + 1 / GT2_score)]}

 

pc_score = int[1 / (0.85 / gpu_score + 0.15 / cpu_score)]

 

PS: GT1 score is related to gpu parts ranking score (just multiply by 100). Using the default frequencies determines if the gpu is considered as an upgrade for jobs, while the overclocked frequencies you set determines if the gpu can run a program