Low is too high, so I'll try and turn it on and off at a frequency that leaves me comfortable...

space_heater.py

#!/usr/bin/env python3

import argparse, atexit, os, subprocess, time
from datetime import datetime

tmpfile = "/tmp/heaterstate"

# it's going to be nicer to have this switch off when the script is killed
def exit_handler():
    set_switch(args.ip, "off")

# argparse
def parse_args():
    parser = argparse.ArgumentParser()
    parser.add_argument("-p", "--percent", default="0", help="Percentage of heater on", type=int)
    parser.add_argument("-i", "--ip", default="10.0.0.19", help="IP address of the heater", type=str)
    return parser.parse_args()

# take percentage and convert it to on/off seconds out of a period of 30 seconds
def calc_secs(percent, period=30):
    on_sec = (percent / 100) * period
    off_sec = (100 - percent) / 100 * period
    return on_sec, off_sec

# pull the single number percentage from storage file
def get_perc(filepath):
    if os.path.exists(filepath):
        with open(filepath) as f:
            perc = f.read()
        return int(perc)

# write the single number percentage to a file
def write_perc(filepath, perc):
    with open(filepath, "w") as f:
        f.write(str(perc))

# set the physical switch
def set_switch(ip, state):
    # obv this would be better if i could figure out how to call natively with python
    subprocess.call(["python3", "ouimeaux/client.py", "--device", ip, "--"+state])
    print(datetime.now(), "Set switch to", state)

# derp main
def main():
    # init
    atexit.register(exit_handler)
    global args
    args = parse_args()
    percentage = args.percent if args.percent>0 else get_perc(tmpfile)
    write_perc(tmpfile, percentage) if args.percent>0 else None
    off, on = calc_secs(percentage)
    print(datetime.now(), "Init state: " , {"off":off, "on":on})
    # loop
    while True:
        percentage = get_perc(tmpfile)
        off, on = calc_secs(percentage)
        set_switch(args.ip, "on")
        time.sleep(on)
        set_switch(args.ip, "off")
        time.sleep(off)

if __name__ == "__main__":
    main()