b - length of the thread (at least)
l - length of Bolts
d - the nominal diameter of the thread
k - head height
e - head width
a - average flow heads

DIN 787 Dimensions for T Slot Bolts

d	a -0.3/-0.5	l-b	l-b	l-b	l-b	e	k
M6	6	25 - 15	40 - 28	63 - 40	-	10-0.5	4-0.5
M8	8	32 - 22	50 - 35	80 - 50	-	13	6
M10	10	40 - 30	63 - 45	100 - 60	-	15	6
M12	12	50 - 35	80 - 55	125 - 75	200 - 120	18	7
M12	14	50 - 35	80 - 55	125 - 75	200 - 120	22	8
M16	18	63 - 45	100 - 63	160 - 100	250 - 150	28	10
M20	22	80 - 55	125 - 85	200 - 125	-	35	14
M24	28	100 - 70	160 - 110	250 - 150	-	44-1	18-1