Determine resistor values when designing astable multivibrator with BJT
In this astable multivibrator



frequency is given by the following formula:

$$
f = \frac 1 {ln(2) \times (R_2C_1 + R_3C_2)} \
\approx \frac {1.443} {(R_2C_1 + R_3C_2)}
$$

If C1 = C2 = C and R2 = R3 = R:

$$
f \approx \frac {0.72} {RC}
$$

It just takes a ratio between R2 and R3 to change the cyclic ratio, keeping $$ \frac {R_2 + R_3} 2 = R $$

R1 and R4 must be low resistances. Their value won't affect frequency if they're noticeably smaller than the base resistors. Just beware of the maximum collector current.

Sources from Wikipedia.