Choose the substitute by function: if you need salt and sheep-milk tang pick Pecorino; for meltability pick young Asiago; for budget pick Grana Padano; for nuttier sheep-milk character pick Manchego.
Measure substitute by weight where possible (1:1). If using volume, grate finely and weigh for accuracy because hard cheeses vary in density.
Adjust seasoning: when using Parmigiano-Reggiano or Grana Padano, add ~¼ teaspoon salt per cup of grated cheese in recipes that rely on Romano for seasoning.
For melting applications, melt gently over medium-low (target sauce ~180–190°F) to avoid separation; young Asiago integrates more smoothly than aged Romano.