I am loading for a 20 tactical. The only loads I have found on the net are for hodgon powder. I am wanting to try IMR 4198. Where would be a starting point with the IMR with the H4198 loads from 23.3 to 26.2. Is there a formula to find a starting load for the next faster or slower if you know a load for another powder close to it?

There is no reliable formula, and loading data should be obtained from a reliable source before starting. That said, the following may help in finding where to look for data.

IMR 4198 may be a hair faster then H4198, but not by much.

Burning Rate Chart (Courtesy of Hodgdon Powder Co.)

(Fastest to Slowest)
1. R-1 Norma
2. N310, Vihtavuori
3. Bullseye, Alliant
4. N312, Vihtavuori
5. Solo 1000, Accurate
6. Clays, Hodgdon
7. Red Dot, Alliant
8. N318, Vihtavuori
9. Hi-Skor 700X, IMR
10. N320, Vihtavuori
11. Green Dot, Alliant
12. International, Hodgdon
13. No. 2, Accurate
14. N321, Vihtavuori
15. N324, Vihtavuori
16. HP-38, Hodgdon
17. W-231, Winchester
18. N325, Vihtavuori
19. N330, Vihtavuori
20. PB, IMR
21. N331, Vihtavuori
22. No. 5, Accurate
23. Unique, Alliant
24. WSL, Winchester
25. Power Pistol, Alliant
26. Universal, Hodgdon
27. SR-7625, IMR
28. W-473AA, Winchester
29. Herco, Alliant
30. N340, Vihtavuori
31. WSF, Winchester
32. HS-6, Hodgdon
33. W-540, Winchester
34. 3N37, Vihtavuori
35. WAP, Winchester
36. Hi-Skor 800-X, IMR
37. N350, Vihtavuori
38. HS-7, Hodgdon
39. W-571, Winchester
40. No. 7, Accurate
41. Blue Dot, Alliant
42. No. 9, Accurate
43. 2400, Alliant
44. N110, Vihtavuori
45. R-123, Norma
46. H-110, Hodgdon
47. W-296, Winchester
48. SR-4759, IMR
49. N120, Vihtavuori
50. XMP-5744, Accurate
51. IMR-4227, IMR
52. N125, Vihtavuori
53. H-4227, Hodgdon
54. N130, Vihtavuori
55. AAC-1680, Accurate
56. W-680, Winchester
57. N132, Vihtavuori
58. N-200, Norma
59. N133, Vihtavuori
60. IMR-4198, IMR
61. H-4198, Hodgdon
62. XMR-2015, Accurate
63. Reloader 7, Alliant
64. N134, Vihtavuori
65. IMR-3031, IMR
66. Benchmark 1, Hodgdon
67. N-201, Norma
68. H-322, Hodgdon
69. Benchmark 2, Hodgdon
70. AAC-2230, Accurate
71. IMR-4895, IMR
72. H-4895, Hodgdon
73. H-335, Hodgdon
74. BL-C(2), Hodgdon
75. AAC-2460, Accurate
76. W-748, Winchester
77. Reloader 12, Alliant
78. N135, Vihtavuori
79. IMR-4064, IMR
80. Varget, Hodgdon
81. AAC-2520, Accurate
82. N-202, Norma
83. XMR-4064, Accurate
84. IMR-4320, IMR
85. N140, Vihtavuori
86. AAC-2700, Accurate
87. Reloader 15, Alliant
88. H-380, Hodgdon
89. N150, Vihtavuori
90. W-760, Winchester
91. H-414, Hodgdon
92. N160, Vihtavuori
93. IMR-4350, IMR
94. H-4350, Hodgdon
95. N-204, Norma
96. Reloader 19, Alliant
97. IMR-4831, IMR
98. XMR-3100, Accurate
99. H-450, Hodgdon
100. H-4831, Hodgdon
101. MRP, Norma
102. N165, Vihtavuori
103. Reloader 22, Alliant
104. IMR-7828, IMR
105. H-1000, Hodgdon
106. XMR-8700, Accurate
107. H-870, Hodgdon
(slowest burn rate)

You might also consider trying Ramshot X-Terminator. I use it in my 20BR. It's accurate and very clean burning. If your rifle is destined for a prairie dog town, it's worth checking out.