It's all about practice. You'll see that players on lvl 50+ have a lot more wins than losses and this is because they learn to calculate these things quickly but also to predict what their opponent is gonna do next.

I'm only level 35 but I already know pretty much every ability in the game off the top of my head so I usually have a plan for the game just after I see my opponent's hand. But when I want to be really, really sure I'm not screwing up my calculations in high pill battles, I open another tab and use Google's calculator.

On regular matches, you have 2 minutes. Make good use of them, but don't use them on purpose to drag the game

For me, I calculate opponent's attack (assuming no attaack manipulation takes place)
Power * (1+Number of pills spent)
so Fei with 7 pillz is 64, for example.
But yea, as Frowns said, it takes experience... what doesn't

Frowns has it right.

With practice you tend to know the general picture of play, like for eg. good players usually pill either 8-9/1 pill(s) on Snowflake, Muze or Cherry. As Frowns has already mentioned above, the more you play the better you get to know about the game, the players and the playstyles. Examples...

... if I happen to fight against a level 25 or below player ... I expect him to either play 6 pills or 3-4 pills in the first round (and which they generally do)
... A Rescue player generally olds or 2 pills each round
... etc

All in all, EVERYONE loses a few matches due to minor miscalculations (Hugo's ability cost me quite a few matches ). The only advice I can give is to play play play and think think think ...

... but please do not think so much in the Fight club or during the DTs. Nobody cares if you win or lose in the Fight club (except for those player fixated on their W/L ratio); and especially during the DTs, even if you do play like a pro, win an impossible match but take 5 minutes to do it ... then you're going straight to your opponents BL (yup ... DT professionals tend to do that)

So ... PLAY PLAY PLAY

Frowns: is correct its all to do with practise (and learning every cards ability/bonus). But even then when im tired i make mistakes (its all a part of this game)

*sigh* There is the chance for you to be concentrated, to learn to calculate quickly and well and to pay attention and what do you do? Ask for a quick fix that automates thinking for you...

(It happens. To everybody, and be it overlooking that Hugo.)

It just takes time, try to remember the staple card abilities, eg. cards like Olga, Spyke, Muze. So on.
If you're just getting used to it open up a calculator before starting a match and refer to it, or atleast freshen up on your 7 and 8 times tables
But yeah, Frowns / scapegod have it right

I just pill randomly and it is still easy 1300 elo every week. Don't over-think it too much unless you are aiming like top 100.

"With practice you tend to know the general picture of play, like for eg. good players usually pill either 8-9/1 pill(s) on Snowflake, Muze or Cherry"
who 9 pillz Cherry?

People who want the nasty 1000 healing mission and maybe also have high DR on top 9-pill the healers. They don't necessarily expect to or want to win, but it CAN be a good choice. (Also depending on what the opponent has)

Then there are Eyrik and Hugo to mess up with the calculations. I hate them.