lotto 6/45 is our country game. ( i can't speak english well. sorry. )

i want to find 4 combination in 28 combination.

if i choose 8 numbers there in correct 6 numbers.
choose 8 number is 1, 2, 3, 4, 5, 6, 7, 8

and i make 28 combination by choose 8 numbers like this ( 8C6 ) :

1, 2, 3, 4, 5, 6
1, 2, 3, 4, 5, 7
1, 2, 3, 4, 5, 8
.
.
.
3, 4, 5, 6, 7, 8
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if i will buy all combination i will be 1st rank.
but i don't want buying all combinations.
i want buy 4 combination guarantee 2nd rank.
1st rank is 6 numbers correct.
2nd rank is 5 numbers correct.

yes i know what is 4 combination.
4 combination is this :
1, 4, 5, 6, 7, 8
2, 3, 4, 5, 6, 7
1, 2, 3, 4, 5, 8
2, 3, 5, 6, 7, 8
-----------------------------

but i don't know fomular how can i find that 8 to 4 combination or 9 to 9 combination or 10 to 16 combination.
please help me.

i just want to know fomular not excel code.

thanks.