Well really all i can say is practice makes perfect. I try to stay no higher then 2500hp or so. DF, chassis, weight are pretty big factors. F1 tires work great but they kinda screw the look of the car. If your buildin a high hp car and dont want to use f1 tires then try editing you .ini file lines to
292=0
296=0
This will give car great handling and breaking. Here is one of my cars that uses this method and is pretty sweet. Other then df maybe grip sometimes, I do everything else in settings.
[GT3CarData]
Region=US
BaseType= r34_gtr_rtune_00
BaseName= NISMO SKYLINE GT-R R-tune (R34,J)
0=1833642646
4=-1706539133
8=-1206
12=-1
16=-1
20=-1
24=-1938339380
28=-1847534766
32=-1929950291
36=-1847534766
40=158979882
44=-1609951638
48=-1937302963
52=-1847534766
56=-1940444788
60=-1847534766
64=-2108084914
68=425751709
72=-1949943446
76=-1847534766
80=-1945720407
84=-1847534766
88=-618228972
92=875966714
96=-1958327944
100=-1847534766
104=-2619186
108=-1
112=-742474717
116=-693812042
120=-1943647635
124=-1847534766
128=-1239608566
132=-897456746
136=-1239608563
140=-313989738
144=-145245973
148=691232822
152=-1239608564
156=-1916639850
160=1321174292
164=-1764196427
168=-1239608569
172=-1987811946
176=-1943598805
180=-1847534766
184=-1936770422
188=-1847534766
192=-1945736787
196=-1847534766
200=-1946248853
204=-1847534766
208=-1934173268
212=-1847534766
216=-12514
220=-1
224=-12514
228=-1
232=-103559162
236=1312048196
240=-2870
244=-2762
248=-1774
252=-1259
256=-941
260=-739
264=-611
268=-16236
272=-4445
276=-40
280=-11
284=-13
288=-13
292=0
296=0
300=-11
304=-41
308=-86
312=-1
316=-1
320=-1
324=-21
328=-11
332=-46
336=-46
340=-129
344=-129
348=-151
352=-161
356=-96
360=-96
364=-8
368=-8
372=-8
376=-8
380=-8
384=-8
388=-8
392=-8
396=-4
400=-5
404=-6
408=-11
412=-11
416=-41
420=-11
424=-21
428=-4445
432=-10
436=-1
440=-1
444=-4
448=-2
452=-2
456=-2
460=-1
464=-1192109520
468=-24236453
472=-6832137
476=-8173
480=933399203
484=933399203
488=0
492=-1
496=-1
500=-1
504=-1
508=-2