calculator for shots per tank

this calculates shots per tank with hpa or nitro...please bear with me on the bad coding, as i just started using DARKBASIC a day ago. the screenshot isnt the bes either, as JPEG decreses quality. PLEASE GIVE ME SUGGESTIONS!

                                    ` This code was downloaded from The Game Creators
                                    ` It is reproduced here with full permission
                                    ` http://www.thegamecreators.com
                                    
                                       ink rgb(0,0,254),1
text 80,200,"NOTE: THIS IS ONLY AN ACCURATE ESTIMATE WITH HPA/NITRO...NOT CO2!"
text 105,215,"THIS ALSO VARYS FROM TANK TO TANK AND MARKER TO MARKER"
wait (5000) : cls : ink rgb(0,0,254),1

   _start:
input "Select tank PSI rating 3000 or 4500>> ";psi$
if psi$="3000" then goto _30k:
if psi$="4500" then goto _45k:

   _45k:
f#=15
g#=15
print " "
PRINT "Enter your tank's size in CUBIC INCHES>"; : input m#
t#=f#*m#
te#=g#*m#
PRINT "===================================================================="
PRINT t#," SHOTS PER TANK" : PRINT " " : PRINT " " : PRINT "Made by: Jarrel Mayer-jerseyjew"
wait (2000) : cls : gosub _exit:

   _30k:
f#=10
g#=10
PRINT "Enter your tank's size in CUBIC INCHES>"; : input m#
t#=f#*m#
te#=g#*m#
PRINT "===================================================================="
PRINT t#," SHOTS PER TANK" : PRINT " " : PRINT " " : PRINT "Made by: Jarrel Mayer-jerseyjew"
wait (2000) : cls : gosub _exit:

   _exit:
do
text 170,200,"             Exit now? y/n"
wait key
if keystate(49)=1 then end
if keystate(21)=1
endif : cls : gosub _start:
loop