Item Lv - % chance of converting into Rank I / II / III / IV Materia:
1-14: 100 / 0 / 0 / 0
15-29: 90 / 10 / 0 / 0
30-44: 0 / 90 / 10 / 0
45-60: 0 / 0 / 90 / 10
60-70: 0 / 0 / 15 / 85
70+: 0 / 0 / 0 / 100

Those should be decently accurate. Note that the higher your iLV gear the longer it takes for 100% spiritbond. It takes 2 or 3 times to spiritbond to 100% gear lv70+ compared to lv45-50 lol