There are generally three ways to approach this: 

1. Channel Loss Budget Analysis
2. Simulate channel s-parameter in the Statistical Eye Analysis Simulator (SeaSim) tool to determine if post-equalized eye height (EH) and eye width (EW) meet the minimum eye opening requirements: ≥6 mV EH and ≥3.13 ps EW at Bit Error Ratio (BER) ≤ 10-6. Refer to PCIe Base Specfication Section 8.5.1.
3. Consider your cost threshold for system upgrades