To bring Venus’s temperature down to between 10 °C and 40 °C, you would need to block between approximately 63% and 76% of the Sun’s rays although due to the significant greenhouse effect, 90% might be more correct. The shield size needed for 90% blockage would be 17,500 km. The ideal point to place the shield is approximately 674,000 km from Venus. Another method is to introduce an organism into the atmosphere, on Earth phytoplankton do this job.