The block load is determined by the angle and pulling force from the line. If the force is coming from the direct opposite side of the attachment point of the block, then the load factor will be 200%. So if you have 100 kg of force in the lines and the angle Φ between them is 0° then the block load will be 200 kg. And if the line force is 150 kg and the angle is 100° then the block load will be 194 kg (150kg * 1,29 = 193,4). Knowing this you just have to make sure you buy a block with a higher safe working load (see block specification).