Crit stat to crit rate questions

I've noticed that a lot of my attacks have been missing, and I need help sorting this out. Does anyone have a crit (the special stat) to crit rate conversion? Is crit even useful to indulge in? Or should my main focus be on increasing dex?
I'm a brawler tank with primarily agility

 

Source: http://www2.hp-ez.com/hp/mobilephoenix/page53

【ＣＲＴ】　　【 クリティカル率 計算ツール 】
・クリティカル率が増加します（＊ｸﾘﾃｨｶﾙﾀﾞﾒｰｼﾞは関係しない）
　→元のクリティカル率は 25
　→CRT3.5 につきクリティカル率が1上昇する
CRTまで考えた栗率基礎値＝25+CRT÷3.5(小数点以下切り捨て)
　クリティカル率＝CRT含む基礎値×（1+0.01×装備クリティカル率+％補正）＋装備クリティカル率+固定値補正

[CRT] [ critical rate calculation tool ]
-critical rate will increase (* critical damage is not related)
　→ original critical rate is 25
　→ critical rate rise 1 per CRT3.5
Critical rate basic values to CRT = 25 + CRT ÷ 3.5 (truncated after the decimal point) 　
crit = CRT including basic value × (1 + 0.01 × equipment critical rate +% correction) + equipment critical rate + fixed value correction

Hope it helps...