https://obsproject.com/estimator
Be aware that non-partnered streamers are advised to not exceed 2000kbps (2mbps), which is enough for a watchable 720p@30fps stream. Also, it is strongly recommended not to exceed 2/3 of your upstream max; about 1250kbps in your case. That includes audio though, which isn't counted in the OBS bitrate setting (handled separately).
Best idea would be to start at 540p@30fps, 1200kbps, with 96kbps AAC audio, and test to see if it will work stably and without dropping frames. On the up side, you should be able to use the Faster or Fast x264 profile with that much extra CPU.
Also, speedtest.net is virtually worthless for livestreamers. It tests a different type of connection metric than livestreaming uses.
You're also going to see significantly higher ping while livestreaming, as it uses up a lot of network throughput. You might drop to 1000kbps to try to combat this a bit, but League is notorious for throwing tantrums when anything 'heavy' is using the connection.